Provide wasm extensions with APIs needed for using pre-installed LSP binaries (#9085)

In this PR, we've added two new methods that LSP extensions can call:
* `shell_env()`, for retrieving the environment variables set in the
user's default shell in the worktree
* `which(command)`, for looking up paths to an executable (accounting
for the user's shell env in the worktree)

To test this out, we moved the `uiua` language support into an
extension. We went ahead and removed the built-in support, since this
language is extremely obscure. Sorry @mikayla-maki. To continue coding
in Uiua in Zed, for now you can `Add Dev Extension` from the extensions
pane, and select the `extensions/uiua` directory in the Zed repo. Very
soon, we'll support publishing these extensions so that you'll be able
to just install it normally.

Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>

crates/extension_api/wit/extension.wit

@@ -61,14 +61,18 @@ world extension {
     /// Updates the installation status for the given language server.
     import set-language-server-installation-status: func(language-server-name: string, status: language-server-installation-status);
 
+    type env-vars = list<tuple<string, string>>;
+
     record command {
         command: string,
         args: list<string>,
-        env: list<tuple<string, string>>,
+        env: env-vars,
     }
 
     resource worktree {
         read-text-file: func(path: string) -> result<string, string>;
+        which: func(binary-name: string) -> option<string>;
+        shell-env: func() -> env-vars;
     }
 
     record language-server-config {