Apply async semantic token modifier to async/await keywords

Only async semantic token modifier

crates/rust-analyzer/src/to_proto.rs

@@ -496,6 +496,7 @@ fn semantic_token_type_and_modifiers(
             HlMod::ControlFlow => semantic_tokens::CONTROL_FLOW,
             HlMod::Mutable => semantic_tokens::MUTABLE,
             HlMod::Consuming => semantic_tokens::CONSUMING,
+            HlMod::Async => semantic_tokens::ASYNC,
             HlMod::Unsafe => semantic_tokens::UNSAFE,
             HlMod::Callable => semantic_tokens::CALLABLE,
             HlMod::Static => lsp_types::SemanticTokenModifier::STATIC,