[emacs] Rework tablegen mode (#182076)
This commit reworks tablegen-mode to be derived from prog-mode and
removes a lot of the manual work that define-derived-mode does for you
these days, along with fixing other lints (such as an over-long
summary).
This is a major version bump because td-decorators-face has been renamed
to tablegen-decorators-face in order to not pollute other namespaces.
