Add auto pairs for same-char pairs

* Add unit tests for all existing functionality
* Add auto pairs for same-char pairs (quotes, etc). Account for
  apostrophe in prose by requiring both sides of the cursor to be
  non-pair chars or whitespace. This also incidentally will work for
  avoiding a double single quote in lifetime annotations, at least until
  <> is added
* Slight factor of moving the cursor transform of the selection to
  inside the hooks. This will enable doing auto pairing with selections,
  and fixing the bug where auto pairs destroy the selection.

Fixes helix-editor#1014