and the magic fixup of opening parentheses.
This commit moves the automatic insertion of matching parens to the
scheme mode keymap, so it will now take affect in Racket mode editing,
only. Also, Rackety.
then, use that to change how it works for the scheme mode (and also another variation for the REPL to
cope with the prompt)
I spent a while trying to make this work at the keymap% level (ie putting different keybindings for "home"
and "c:a" into different keymaps) but this just turned out to be far too confusing and fragile, so went
with this alternative (one keybinding, but that delegates to an overridable method)
closes PR 11446
