Replace tabstop keymap handling with a minor mode / overlay

[the-mikedavis]

The snippet tabstop PR #9801 added some minor ad-hoc keymap handling code to allow transitioning between tabstops. Some early review mentioned using minor modes as a sort of overlay on the current keymap to overwrite the tab binding instead #9801 (comment). This is preferrable but larger in scope than the code added in #9801. We should replace the ad-hoc stuff with an overlay/minor-mode once we refactor minor modes to be capable of layering on other modes.