After #38882 we were always including file/directory mentions as
`zed:///agent/file?path=a/b/c.rs`.
However, for most resource links (files/directories/symbols/selections)
we want to use a common format, so that ACP servers don't have to
implement custom handling for parsing `ResourceLink`s coming from Zed.
This is what it looks like now:
```
[@index.js](file:///Users/.../projects/reqwest/examples/wasm_github_fetch/index.js)
[@wasm](file:///Users/.../projects/reqwest/src/wasm)
[@Error](file:///Users/.../projects/reqwest/src/async_impl/client.rs?symbol=Error#L2661:2661)
[@error.rs (23:27)](file:///Users/.../projects/reqwest/src/error.rs#L23:27)
```
Release Notes:
- N/A
---------
Co-authored-by: Cole Miller <cole@zed.dev>
2579f86bcd