fix: validate positions on delete and move

[chentoast]

When the buffer state changes, the internal mark list diverges from the actual mark positions. So, recompute mark positions before performing a delete or move.

[chentoast]

Going to merge this as a temporary fix. There are still a couple of bugs with changing buffer state that will need a more principled fix:

• restoring marks when undoing a delete
• clearing signs for deletes with signcolumn=yes:[0-9]
• moving signs for deletes on bookmark
• Incorrect behavior in MarksList*.

I think an implementation based on nvim_buf_attach should be able to solve these problems.