Support opening and saving files with legacy encodings (#44819)

## Summary

Addresses #16965

This PR adds support for **opening and saving** files with legacy
encodings (non-UTF-8).
Previously, Zed failed to open files encoded in Shift-JIS, EUC-JP, Big5,
etc., displaying a "Could not open file" error screen. This PR
implements automatic encoding detection upon opening and ensures the
original encoding is preserved when saving.

## Implementation Details

1.  **Worktree (Loading)**:
* Updated `load_file` to use `chardetng` for automatic encoding
detection.
* Files are decoded to UTF-8 internal strings for editing, while
preserving the detected `Encoding` metadata.
2.  **Language / Buffer**:
* Added an `encoding` field to the `Buffer` struct to store the detected
encoding.
3.  **Worktree (Saving)**:
    * Updated `write_file` to accept the stored encoding.
    * **Performance Optimization**:
* **UTF-8 Path**: Uses the existing optimized `fs.save` (streaming
chunks directly from Rope), ensuring no performance regression for the
vast majority of files.
* **Legacy Encoding Path**: Implemented a fallback that converts the
Rope to a contiguous `String/Bytes` in memory, re-encodes it to the
target format (e.g., Shift-JIS), and writes it to disk.
* *Note*: This fallback involves memory allocation, but it is necessary
to support legacy encodings without refactoring the `fs` crate's
streaming interfaces.

## Changes

- `crates/worktree`:
    - Add dependencies: `encoding_rs`, `chardetng`.
    - Update `load_file` to detect encoding and decode content.
    - Update `write_file` to handle re-encoding on save.
- `crates/language`: Add `encoding` field and accessors to `Buffer`.
- `crates/project`: Pass encoding information between Worktree and
Buffer.
- `crates/vim`: Update `:w` command to use the new `write_file`
signature.

## Verification

I validated this manually using a Rust script to generate test files
with various encodings.

**Results:**

* ✅ **Success (Opened & Saved correctly):**
    * **Japanese:** `Shift-JIS` (CP932), `EUC-JP`, `ISO-2022-JP`
    * **Chinese:** `Big5` (Traditional), `GBK/GB2312` (Simplified)
* **Western/Unicode:** `Windows-1252` (CP1252), `UTF-16LE`, `UTF-16BE`
* ⚠️ **limitations (Detection accuracy):**
* Some specific encodings like `KOI8-R` or generic `Latin1` (ISO-8859-1)
may partially display replacement characters (`?`) depending on the file
content length. This is a known limitation of the heuristic detection
library (`chardetng`) rather than the saving logic.


Release Notes:

- Added support for opening and saving files with legacy encodings
(Shift-JIS, Big5, etc.)

---------

Co-authored-by: CrazyboyQCD <53971641+CrazyboyQCD@users.noreply.github.com>
Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

Cargo.lock

@@ -2667,9 +2667,9 @@ dependencies = [
 
 [[package]]
 name = "cap-fs-ext"
-version = "3.4.5"
+version = "3.4.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d5528f85b1e134ae811704e41ef80930f56e795923f866813255bc342cc20654"
+checksum = "e41cc18551193fe8fa6f15c1e3c799bc5ec9e2cfbfaa8ed46f37013e3e6c173c"
 dependencies = [
  "cap-primitives",
  "cap-std",
@@ -2679,9 +2679,9 @@ dependencies = [
 
 [[package]]
 name = "cap-net-ext"
-version = "3.4.5"
+version = "3.4.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "20a158160765c6a7d0d8c072a53d772e4cb243f38b04bfcf6b4939cfbe7482e7"
+checksum = "9f83833816c66c986e913b22ac887cec216ea09301802054316fc5301809702c"
 dependencies = [
  "cap-primitives",
  "cap-std",
@@ -2691,9 +2691,9 @@ dependencies = [
 
 [[package]]
 name = "cap-primitives"
-version = "3.4.5"
+version = "3.4.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b6cf3aea8a5081171859ef57bc1606b1df6999df4f1110f8eef68b30098d1d3a"
+checksum = "0a1e394ed14f39f8bc26f59d4c0c010dbe7f0a1b9bafff451b1f98b67c8af62a"
 dependencies = [
  "ambient-authority",
  "fs-set-times",
@@ -2709,9 +2709,9 @@ dependencies = [
 
 [[package]]
 name = "cap-rand"
-version = "3.4.5"
+version = "3.4.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d8144c22e24bbcf26ade86cb6501a0916c46b7e4787abdb0045a467eb1645a1d"
+checksum = "0acb89ccf798a28683f00089d0630dfaceec087234eae0d308c05ddeaa941b40"
 dependencies = [
  "ambient-authority",
  "rand 0.8.5",
@@ -2719,9 +2719,9 @@ dependencies = [
 
 [[package]]
 name = "cap-std"
-version = "3.4.5"
+version = "3.4.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b6dc3090992a735d23219de5c204927163d922f42f575a0189b005c62d37549a"
+checksum = "07c0355ca583dd58f176c3c12489d684163861ede3c9efa6fd8bba314c984189"
 dependencies = [
  "cap-primitives",
  "io-extras",
@@ -2731,9 +2731,9 @@ dependencies = [
 
 [[package]]
 name = "cap-time-ext"
-version = "3.4.5"
+version = "3.4.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "def102506ce40c11710a9b16e614af0cde8e76ae51b1f48c04b8d79f4b671a80"
+checksum = "491af520b8770085daa0466978c75db90368c71896523f2464214e38359b1a5b"
 dependencies = [
  "ambient-authority",
  "cap-primitives",
@@ -2896,6 +2896,17 @@ dependencies = [
  "util",
 ]
 
+[[package]]
+name = "chardetng"
+version = "0.1.17"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "14b8f0b65b7b08ae3c8187e8d77174de20cb6777864c6b832d8ad365999cf1ea"
+dependencies = [
+ "cfg-if",
+ "encoding_rs",
+ "memchr",
+]
+
 [[package]]
 name = "chrono"
 version = "0.4.42"
@@ -8797,6 +8808,7 @@ dependencies = [
  "ctor",
  "diffy",
  "ec4rs",
+ "encoding_rs",
  "fs",
  "futures 0.3.31",
  "fuzzy",
@@ -12465,6 +12477,7 @@ dependencies = [
  "dap",
  "dap_adapters",
  "db",
+ "encoding_rs",
  "extension",
  "fancy-regex",
  "fs",
@@ -20231,8 +20244,10 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
  "async-lock 2.8.0",
+ "chardetng",
  "clock",
  "collections",
+ "encoding_rs",
  "fs",
  "futures 0.3.31",
  "fuzzy",