feat: script module flag

The embed request now says whether JavaScript is a module or a
classic script, as Prettier's __babelSourceType does: a <script> in
HTML is a classic script unless its type makes it a module. Legacy
<!-- --> comments only parse in a classic script.

A </script in the host's answer would end the element early, so it
is escaped as <\/script, which Prettier also prints in the one place
where one can come out.