Closes https://github.com/zed-industries/zed/issues/41407

This PR fixes the issues that caused #41407 to be reverted in #42008.
Namely that the action context didn't take into account if a snippet
could move backwards or forwards, and the action shared the same key
mapping as `editor::MoveToPreviousWordStart` and
`editor::MoveToNextWordEnd`.

I changed the default key mapping for the move to snippet tabstop to tab
and shift-tab to match the default behavior of other editors.

Release Notes:

- Editor: Add actions to move between snippet tabstop positions