Assign buffer's completion triggers from LSP capabilities

Also, make LanguageServer::new() async. The future resolves
once the server is initialized.

crates/project/src/worktree.rs

@@ -556,7 +556,6 @@ impl LocalWorktree {
     }
 
     pub fn diagnostics_for_path(&self, path: &Path) -> Option<Vec<DiagnosticEntry<PointUtf16>>> {
-        dbg!(&self.diagnostics);
         self.diagnostics.get(path).cloned()
     }