Map more UTF-8 key events

At the moment we are only handling the space " " and keys in a nice way, but it'd be nice to be able to match on KeyDown Enter or KeyDown "<enter>".

Using a variant for all the keys may be a stretch, since we're parsing UTF-8 grapheme clusters, so at least some common ones we want to handle in a way that lets you match over KeyDown easily.

This is currently happening in the minttea/io_loop.ml in the translate function:

https://github.com/leostera/minttea/blob/main/minttea/io_loop.ml#L18

Currently implemented keys:

• Enter
• Space
• Backspace
• Left / Down / Up / Right
• Escape
• Key (includes a string with the contents of the matched UTF-8 character)

Missing keys:

• Shift-modifier
• Ctrl-modifier
• Alt-modifier
• ???

Start in minttea/io_loop.ml at the translate function and inspect how the currently implemented key events are represented and matched. Compare the existing Enter, Space, Backspace, arrow, Escape, and UTF-8 key handling with the missing modifier cases. Done means the intended common key and modifier events can be matched consistently, with tests covering the added mappings.