T2: use editor.formatOnSave and rust.build_on_save

[yangwenmai]

editor.formatOnSave

保存 rust 文件即可实现 rustfmt；

rust.build_on_save

保存 rust 文件即可 build；

参考资料

1. editor.formatOnSave does not actually format on save with RLS editor-rs/vscode-rust#253
2. https://github.com/editor-rs/vscode-rust/blob/master/doc/format.md