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.lock

@@ -8134,6 +8134,7 @@ checksum = "d3f763c1041eff92ffb5d7169968a327e1ed2ebfe425dac0ee5a35f29082534b"
 dependencies = [
  "bstr",
  "either",
+ "futures-util",
  "mlua-sys",
  "num-traits",
  "parking_lot",
@@ -11915,12 +11916,17 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
  "assistant_tool",
+ "futures 0.3.31",
  "gpui",
  "mlua",
+ "parking_lot",
+ "project",
  "regex",
  "schemars",
  "serde",
  "serde_json",
+ "smol",
+ "util",
  "workspace",
 ]