Fix zero-width regex replacements

[lawrence3699]

Fixes #775.

Replacing a zero-width regex match currently relies on having an active selection.
That works for normal matches, but patterns like $ produce an empty match, so the selection is cleared and both Replace and Replace All turn into no-ops.

This change keeps track of the active regex match separately from the visible selection so zero-width matches can still be replaced.
After replacing an empty match, Replace All now advances past the next grapheme (or forces a wrap at EOF) so it does not get stuck on the same logical position.
The cached zero-width match is also ignored once the cursor moves away, so Replace does not insert at a stale location.

Validation:

• cargo fmt --all
• cargo test -p edit

[lhecker]

We've received two PRs about within a few days of each other. I believe #815 is unfortunately superior so I'll be closing this PR for now. That said, I'm not sure I'm happy with #815 either.