Add async and await keywords to Rust highlighter

This commit adds support for Rust's `async` and `await` keywords to `language-rust-bundled`. See atom#20905 for more information.

Author: slightknack

packages/language-rust-bundled/grammars/tree-sitter-rust.cson

@@ -131,6 +131,8 @@ scopes:
   '"where"': 'keyword.control'
   '"ref"': 'keyword.control'
   '"macro_rules!"': 'keyword.control'
+  '"async"': 'keyword.control'
+  '"await"': 'keyword.control'
 
   '"as"': 'keyword.operator'