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/language/src/language.rs

@@ -40,7 +40,7 @@ use smol::future::FutureExt as _;
 use std::{
     any::Any,
     cell::RefCell,
-    ffi::OsString,
+    ffi::OsStr,
     fmt::Debug,
     hash::Hash,
     mem,
@@ -277,7 +277,8 @@ pub trait LspAdapterDelegate: Send + Sync {
     fn http_client(&self) -> Arc<dyn HttpClient>;
     fn update_status(&self, language: LanguageServerName, status: LanguageServerBinaryStatus);
 
-    async fn which_command(&self, command: OsString) -> Option<(PathBuf, HashMap<String, String>)>;
+    async fn which(&self, command: &OsStr) -> Option<PathBuf>;
+    async fn shell_env(&self) -> HashMap<String, String>;
     async fn read_text_file(&self, path: PathBuf) -> Result<String>;
 }