Need to flag mixing up "of" and "off"

This is common even among native speakers. It's often a spelling mistake by people that don't always know which is which, but it's probably a typo by people who do know but their fingers did the wrong thing.

We do address this in some idiomatic uses such as "of course" vs "off course", but it can happen anywhere.

Real world examples:
- It would be nice if the extension could turn **of** for some pages automatically and stay on most of the time.

Start by locating the existing spelling or grammar rules that handle similar word confusions, then trace how their diagnostics are tested. Add coverage for “of” and “off” in the reported example and comparable contexts, and confirm that valid uses of both words remain unflagged.

Written by the indexing model from the issue text.