Create `OnOffToggle` component for preferences.

[lindapaiste]

This is purely a code cleanup, there is no change to UI or behavior. It extracts a lot of repetitive code in the Preferences component into a new component OnOffToggle.

The component is inspired by @ghalestrilo's optionsOnOff code for the mobile preferences (which will be going away, in favor of a single responsive component).

It will probably evolve more as I refactor other parts of the Preferences.

Changes:

• Create a component OnOffToggle for the clickable "On" and "Off" labels.
• Write unit tests.
• Use the OnOffToggle in Preferences for Autosave, Autoclose Brackets and Quotes, etc.

I have verified that this pull request:

• [x] has no linting errors (npm run lint)
• [x] has no test errors (npm run test)
• [x] is from a uniquely-named feature branch and is up to date with the develop branch.
• [ ] is descriptively named and links to an issue number, i.e. Fixes #123

[lindapaiste]

Maybe I should rework the PreferencePicker instead of making a whole new component? 🤷

[lindapaiste]

Possibly translationKey="LineNumbers" is a bit too "tricky" and it would be be better to be more explicit, even though it's more verbose. I can change the syntax to ariaLabelOn={t('Preferences.LineNumbersOnARIA')} ariaLabelOff={t('Preferences.LineNumbersOffARIA')}.