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