Highlight Rust async and await as keywords

[coyaSONG]

Summary

• extend bat's existing Rust syntax patch so async and await use the control-keyword scope
• add the issue's async function and chained .await expression to the Rust syntax fixture
• commit the regenerated highlighted fixture while leaving generated binary asset caches out of the pull request

Closes #1534.

Root cause

bat's pinned Sublime Packages revision predates Rust async/await syntax, so both tokens fell through to ordinary identifier styling.

Validation

• verified the updated patch applies cleanly to the exact pinned syntax source
• ran the full repository asset builder successfully
• rebuilt bat and confirmed async/await render with the Rust control-keyword color while async_foo remains a function identifier
• confirmed the committed Rust fixture is byte-identical to a fresh focused render
• cargo fmt --check
• cargo test --locked
• git diff --check

AI assistance

OpenAI Codex was used to inspect the issue and repository guidance, implement the change, and run the checks listed above.