feature: apply (fold & unfold) substitutions to selection

[siegebell]

Given a selection, provide commands:

• Prettify symbols mode: fold selection: apply the substitutions to the text itself
• Prettify symbols mode: unfold selection: attempt to replace pretties (appearing as actual text) in the selection with their ugly counterpart. Only unfold a pretty when: its ugly pre-, post-, and scope- condition will be satisfied, and only if there is exactly one ugly that could apply. Multiple rules can be used to unfold the pretty so long as the resulting ugly text is the same.