feat: indicate misspelled word under cursor with pale error colour background...

feat: indicate misspelled word under cursor with pale error colour background instead of messing with selection

Users generally aren't a fan of the computer modifiying something that should be under their control without their involvement, and the current behaviour has generated large amounts of complaints from Tok users (until I removed needed code for this behaviour from Tok).

Tracking selection ourselves and using a pale error colour highlight results in both nicer aesthetics and resolving the behavioural issue users keep complaining about.