[feature] add setting to optionally show invisible characters

Nobody has claimed this yet.

Discussion in issue #88 raised the point that there might be value in adding a setting that enables people to show invisible characters.

I think this would be a good feature to add with the following considerations:
- it should be off by default. There are many people who dislike having invisible characters displayed.
- the invisible characters should be displayed in a subtle manner.

I mention the subtle part because after years of staring at invisible characters I've found the most useful display to be one where they are visible, but subtle enough that they can be easily ignored.

I'd also suggest only displaying tabs and spaces, not any of the other invisible chars.

No files, tests, or entry points are named. Start by reading issue #88 and locating the existing settings and editor-rendering entry points; done means an opt-in setting displays only tabs and spaces subtly while remaining off by default.

Written by the indexing model from the issue text.