Closes #ISSUE
[git: Use font size to determine pattern slash width
#26446](https://github.com/zed-industries/zed/pull/26446)
This PR only uses font size as the slant line width, and here it further
uses line height as the slant line interval control.
before

now

big line height

Release Notes:
- N/A *or* Added/Fixed/Improved ...
Release Notes:
- Render unstaged hunks in the project diff editor with a slashed
background
---------
Co-authored-by: maxbrunsfeld <max@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
TODO:
- [x] BackgroundOrientation
- [x] PatternDash
- [x] `pattern_horizontal_dash` & `pattern_vertical_dash`
- [x] Metal dash shader
- [x] Blade dash shader
- [x] Update ui::Divider to use new pattern
---
This PR introduces proper dashed dividers using the new `PatternDash`
background shader.

Before this we were using 128 elements to create a dashed divider, which
is both expensive, and would not scale beyond a certain size. This
allows us to simplify the divider element as well.
Changes:
- Adds `BackgroundOrientation` to `gpui::color::Background` to allow
specifying a direction for a pattern
- Adds the PatternDash pattern variant
- Updates `ui::Divider`'s dashed variants to be more efficient
Misc:
- Documents the `ui::Divider` component
- Treat `.metal` files as `C` in the Zed project until we get some metal
syntax highlighting.
Release Notes:
- N/A
TODO:
- [x] Add BackgroundTag::PatternSlash
- [x] Support metal slash pattern fills
- [x] Support blade slash pattern fills
---
Adds support for a new background type in gpui, `pattern_slash`.
Usage:
```rust
div().size(px(56.0)).bg(pattern_slash(gpui::red()))
```
This will create a 56px square with a red slash pattern fill.
You can run the pattern example with `cargo run -p gpui --example
pattern`:

---
After talking with @as-cii at length about how we want to support
patterns in gpui, we decided for now we'll simply add a new
BackgroundTag specific to this pattern.
It isn't the best long term plan however – we'll likely want to
introduce the concept of a `Fill` at some point so we can have
`Fill::Solid`, `Fill::Gradient(LinearGradient)`, etc in the future.
The pattern is designed to seamlessly tile vertically for elements of
the same height. For example, for use in editor line backgrounds:

---
Release Notes:
(do we do gpui release notes?)
- Adds support for slash pattern fills in `gpui`.
---------
Co-authored-by: Antonio Scandurra <me@as-cii.com>