Add initial support for defining language server adapters in WebAssembly-based extensions (#8645)

This PR adds **internal** ability to run arbitrary language servers via
WebAssembly extensions. The functionality isn't exposed yet - we're just
landing this in this early state because there have been a lot of
changes to the `LspAdapter` trait, and other language server logic.

## Next steps

* Currently, wasm extensions can only define how to *install* and run a
language server, they can't yet implement the other LSP adapter methods,
such as formatting completion labels and workspace symbols.
* We don't have an automatic way to install or develop these types of
extensions
* We don't have a way to package these types of extensions in our
extensions repo, to make them available via our extensions API.
* The Rust extension API crate, `zed-extension-api` has not yet been
published to crates.io, because we still consider the API a work in
progress.

Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Nathan <nathan@zed.dev>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/language/src/proto.rs

@@ -2,7 +2,7 @@
 
 use crate::{
     diagnostic_set::DiagnosticEntry, CodeAction, CodeLabel, Completion, CursorShape, Diagnostic,
-    Language,
+    Language, LanguageRegistry,
 };
 use anyhow::{anyhow, Result};
 use clock::ReplicaId;
@@ -487,6 +487,7 @@ pub fn serialize_completion(completion: &Completion) -> proto::Completion {
 pub async fn deserialize_completion(
     completion: proto::Completion,
     language: Option<Arc<Language>>,
+    language_registry: &Arc<LanguageRegistry>,
 ) -> Result<Completion> {
     let old_start = completion
         .old_start
@@ -500,7 +501,11 @@ pub async fn deserialize_completion(
 
     let mut label = None;
     if let Some(language) = language {
-        label = language.label_for_completion(&lsp_completion).await;
+        if let Some(adapter) = language_registry.lsp_adapters(&language).first() {
+            label = adapter
+                .label_for_completion(&lsp_completion, &language)
+                .await;
+        }
     }
 
     Ok(Completion {