Treat autocompletions as discrete tokens

[brandonpayton]

This is an issue for discussing and tracking work to strengthen our treatment of autocompletions.

• Insert autocompletions as tokens, discrete units embedded within the text.
• Support styling autocomplete tokens per completer.
• Style core autocompleter tokens.
• Ensure token insertion is included in the undo stack.

[brandonpayton]

I'm going to close this it's not going to happen for 5.0 We can resurrect post-5.0 if desired.