Simplify livekit config so that cargo check Just Works (#21661)

Supersedes https://github.com/zed-industries/zed/pull/21653

This enables us to use `cargo test -p workspace` on macOS and Linux.

Note that the line diffs in `shared_screen.rs` are spurious, I just
re-ordered the `macos` and `cross-platform` modules to match the order
in the call crate.

Release Notes:

- N/A

typos.toml

@@ -43,6 +43,8 @@ extend-exclude = [
     "docs/theme/css/",
     # Spellcheck triggers on `|Fixe[sd]|` regex part.
     "script/danger/dangerfile.ts",
+    # Hashes are not typos
+    "script/patches/use-cross-platform-livekit.patch"
 ]
 
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