Add a feature to map left Option `⌥` key to Meta

Is your feature request about something that is currently impossible or hard to do? Please describe the problem.

I don't really use accented characters using the macOS compose key. So I like to treat the left Option key as an additional modifier for more key mapping space. Creating these mappings for the text version of vim running inside of iTerm works great because I have the setting "Left Option key: Esc+".

The problem is that in MacVim, the Left Option key continues to act like the macOS Compose key. That means that I have to add to my .vimrc a bunch of mappings like:

nmap ß <M-s>
imap ß <M-s>

Describe the solution you'd like
Like VimR, let me map the left Option key as Meta:

Describe alternatives you've considered
Add a bunch of alias mappings like:

nmap ß <M-s>
imap ß <M-s>

Issue では MacVim、VimR、.vimrc のマッピングが挙げられていますが、ソースファイルやテストは示されていません。まず MacVim の macOS キーボード処理を見つけ、VimR が左 Option キーをどのように扱っているかを比較します。左 Option キーを、記載されているエイリアスマッピングを必要とせずに Meta として設定できれば完了です。