Simplify language server startup (#6449)

These are just some small refactorings of our language-server-starting
code, motivated by another change that I decided to bail on:
https://github.com/zed-industries/zed/pull/6448.

crates/zed/src/languages/lua.rs

@@ -18,7 +18,7 @@ pub struct LuaLspAdapter;
 
 #[async_trait]
 impl super::LspAdapter for LuaLspAdapter {
-    async fn name(&self) -> LanguageServerName {
+    fn name(&self) -> LanguageServerName {
         LanguageServerName("lua-language-server".into())
     }