refactor: Text editing

[Tropix126]

This is a best-effort refactor of text editing. It's currently very unfinished (not up to functionality of previous implementation).

Keyboard Interactions

• Arrow Keys
  • Navigate singular characters and lines
  • Ctrl/Meta modifier
  • Shift modifier
• Backspace
  • Delete signular characters, unwrapping lines if necessary.
  • Ctrl/Meta modifier
• Delete
  • Delete character ahead of cursor
  • Ctrl/Meta modifier
• Home
  • Jump to start of line
  • Ctrl/Meta modifier (Jump to start of buffer)
  • Shift modifier
• End
  • Jump to end of line
  • Ctrl/Meta modifier (Jump to end of buffer)
  • Shift modifier
• Ctrl + A
• Ctrl + L (Maybe)
• PageUp
• PageDown
• The other things listed in https://www.w3schools.com/tags/ref_keyboardshortcuts.asp

Mouse Interactions

• Dragging (broken)
• Double click to select with grapheme granularity

Other Stuff

• IME Support

Cursor Features

• Grapheme/word jumping with unicode_segmentation crate.
• Affinity (required for bidirectional text support)
• Remember x-position when line jumping

[Tropix126]

Some things (cursor x-position and affinity) might be better suited for a future PR, but i'll move that into its own tracking issue once things like bidirectional text and RTL become a concern.