Add a settings page

Over time, more and more things may want to be configurable. Today it's:

• Theme colors
• Changing keyboard bindings
• Setting the ruler column for COMMIT_EDITMSG and others
• Enabling/Disabling the margin (line numbers)
• Enabling word-wrap for certain file types
• Cursor style
• Application language

We should use TOML, because JSON is kind of meh. It's also used by Helix.

On top of adding the mere dialog, this feature requires implementing settings persistence. This would mean:

• Windows: A settings file
  • BUT: There are concerns about this making GPO more difficult. This needs to be watertight before we ship it.
• Linux: XDG ~/.config directory - maybe as a JSON file?
  • Implementing a JSON parser by hand would be funny
• macOS: Settings file but not sure which path ❔