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

crates/scripting_tool/Cargo.toml

@@ -15,10 +15,15 @@ doctest = false
 [dependencies]
 anyhow.workspace = true
 assistant_tool.workspace = true
+futures.workspace = true
 gpui.workspace = true
 mlua.workspace = true
+parking_lot.workspace = true
+project.workspace = true
+regex.workspace = true
 schemars.workspace = true
 serde.workspace = true
 serde_json.workspace = true
+smol.workspace = true
+util.workspace = true
 workspace.workspace = true
-regex.workspace = true