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>

extensions/uiua/src/uiua.rs

@@ -0,0 +1,27 @@
+use zed_extension_api::{self as zed, Result};
+
+struct UiuaExtension;
+
+impl zed::Extension for UiuaExtension {
+    fn new() -> Self {
+        Self
+    }
+
+    fn language_server_command(
+        &mut self,
+        _config: zed::LanguageServerConfig,
+        worktree: &zed::Worktree,
+    ) -> Result<zed::Command> {
+        let path = worktree
+            .which("uiua")
+            .ok_or_else(|| "uiua is not installed".to_string())?;
+
+        Ok(zed::Command {
+            command: path,
+            args: vec!["lsp".to_string()],
+            env: Default::default(),
+        })
+    }
+}
+
+zed::register_extension!(UiuaExtension);