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/src/extension_store_test.rs

@@ -7,10 +7,7 @@ use collections::BTreeMap;
 use fs::{FakeFs, Fs, RealFs};
 use futures::{io::BufReader, AsyncReadExt, StreamExt};
 use gpui::{Context, TestAppContext};
-use language::{
-    Language, LanguageConfig, LanguageMatcher, LanguageRegistry, LanguageServerBinaryStatus,
-    LanguageServerName,
-};
+use language::{LanguageMatcher, LanguageRegistry, LanguageServerBinaryStatus, LanguageServerName};
 use node_runtime::FakeNodeRuntime;
 use parking_lot::Mutex;
 use project::Project;
@@ -573,19 +570,6 @@ async fn test_extension_store_with_gleam_extension(cx: &mut TestAppContext) {
         })
         .await
         .unwrap();
-    project.update(cx, |project, cx| {
-        project.set_language_for_buffer(
-            &buffer,
-            Arc::new(Language::new(
-                LanguageConfig {
-                    name: "Gleam".into(),
-                    ..Default::default()
-                },
-                None,
-            )),
-            cx,
-        )
-    });
 
     let fake_server = fake_servers.next().await.unwrap();
     let expected_server_path = extensions_dir.join("work/gleam/gleam-v1.2.3/gleam");