Allow controlling Tailwind via the language_servers setting (#11012)

This PR adds the ability for the Tailwind language server
(`tailwindcss-language-server`) to be controlled by the
`language_servers` setting.

Now in your settings you can indicate that the Tailwind language server
should be used for a given language, even if that language does not have
the Tailwind language server registered for it already:

```json
{
  "languages": {
    "My Language": {
      "language_servers": ["tailwindcss-language-server", "..."]
    }
  }
}
```

Release Notes:

- N/A

crates/language/src/language_registry.rs

@@ -46,6 +46,8 @@ struct LanguageRegistryState {
     available_languages: Vec<AvailableLanguage>,
     grammars: HashMap<Arc<str>, AvailableGrammar>,
     lsp_adapters: HashMap<Arc<str>, Vec<Arc<CachedLspAdapter>>>,
+    available_lsp_adapters:
+        HashMap<LanguageServerName, Arc<dyn Fn() -> Arc<CachedLspAdapter> + 'static + Send + Sync>>,
     loading_languages: HashMap<LanguageId, Vec<oneshot::Sender<Result<Arc<Language>>>>>,
     subscription: (watch::Sender<()>, watch::Receiver<()>),
     theme: Option<Arc<Theme>>,
@@ -153,6 +155,7 @@ impl LanguageRegistry {
                 language_settings: Default::default(),
                 loading_languages: Default::default(),
                 lsp_adapters: Default::default(),
+                available_lsp_adapters: HashMap::default(),
                 subscription: watch::channel(),
                 theme: Default::default(),
                 version: 0,
@@ -213,6 +216,38 @@ impl LanguageRegistry {
         )
     }
 
+    /// Registers an available language server adapter.
+    ///
+    /// The language server is registered under the language server name, but
+    /// not bound to a particular language.
+    ///
+    /// When a language wants to load this particular language server, it will
+    /// invoke the `load` function.
+    pub fn register_available_lsp_adapter(
+        &self,
+        name: LanguageServerName,
+        load: impl Fn() -> Arc<dyn LspAdapter> + 'static + Send + Sync,
+    ) {
+        self.state.write().available_lsp_adapters.insert(
+            name,
+            Arc::new(move || {
+                let lsp_adapter = load();
+                CachedLspAdapter::new(lsp_adapter, true)
+            }),
+        );
+    }
+
+    /// Loads the language server adapter for the language server with the given name.
+    pub fn load_available_lsp_adapter(
+        &self,
+        name: &LanguageServerName,
+    ) -> Option<Arc<CachedLspAdapter>> {
+        let state = self.state.read();
+        let load_lsp_adapter = state.available_lsp_adapters.get(name)?;
+
+        Some(load_lsp_adapter())
+    }
+
     pub fn register_lsp_adapter(&self, language_name: Arc<str>, adapter: Arc<dyn LspAdapter>) {
         self.state
             .write()

crates/language/src/language_settings.rs

@@ -789,5 +789,24 @@ mod tests {
             ),
             language_server_names(&["deno", "eslint", "tailwind"])
         );
+
+        // Adding a language server not in the list of available languages servers adds it to the list.
+        assert_eq!(
+            LanguageSettings::resolve_language_servers(
+                &[
+                    "my-cool-language-server".into(),
+                    LanguageSettings::REST_OF_LANGUAGE_SERVERS.into()
+                ],
+                &available_language_servers
+            ),
+            language_server_names(&[
+                "my-cool-language-server",
+                "typescript-language-server",
+                "biome",
+                "deno",
+                "eslint",
+                "tailwind",
+            ])
+        );
     }
 }