Add lua script access to code using cx + reuse project search logic (#26269)

Access to `cx` will be needed for anything that queries entities. In
this commit this is use of `WorktreeStore::find_search_candidates`. In
the future it will be things like access to LSP / tree-sitter outlines /
etc.

Changes to support access to `cx` from functions provided to the Lua
script:

* Adds a channel of requests that require a `cx`. Work enqueued to this
channel is run on the foreground thread.

* Adds `async` and `send` features to `mlua` crate so that async rust
functions can be used from Lua.

* Changes uses of `Rc<RefCell<...>>` to `Arc<Mutex<...>>` so that the
futures are `Send`.

One benefit of reusing project search logic for search candidates is
that it properly ignores paths.

Release Notes:

- N/A

Cargo.toml

@@ -452,7 +452,7 @@ livekit = { git = "https://github.com/zed-industries/livekit-rust-sdks", rev = "
 ], default-features = false }
 log = { version = "0.4.16", features = ["kv_unstable_serde", "serde"] }
 markup5ever_rcdom = "0.3.0"
-mlua = { version = "0.10", features = ["lua54", "vendored"] }
+mlua = { version = "0.10", features = ["lua54", "vendored", "async", "send"] }
 nanoid = "0.4"
 nbformat = { version = "0.10.0" }
 nix = "0.29"