balance-parens in scheme:text<%>
This function is called when the user types a close parenthesis in the
text%. If the close parenthesis that the user inserted does not match the
corresponding open parenthesis and the 'framework:fixup-parens preference is
#t (see
preferences:get) the correct closing parenthesis is inserted.
If the 'framework:paren-match preference is
#t (see
preferences:get) the matching open parenthesis is flashed.
(-> voidsenda-scheme:textbalance-parenskey-event)
key-event: (instancekey-event%)