Add a full set of CC Mode language variables to js.el.

Add a full set of CC Mode language variables to js.el.

This will allow js.el to work after maintenance changes in CC Mode, e.g. the
fix to bug #11165.

* lisp/progmodes/js.el (top level): Create a CC Mode derived language called
js-mode and based on Java Mode.
(js-mode): call c-init-language-vars for js-mode.  Remove the direct settings
of several CC Mode language variables which are now defined through the
derived language mechanism.  Call c-set-style and set up the needed style
variables c-block-comment-prefix and c-comment-prefix-regexp.