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.

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