Another issue if we want to expand the scope of electric-pair is that
when the users type `case` maybe they're just yping `case` but cases
they're actually on their way to typing `cases` or `cased` or `case_nb`
or god knows what else: for those matched thingies that can be prefixes of
arbitrary identifiers, we'd probably want to wait for the user to hit
SPC (like abbrev does) before inserting the closing element.