Add application/javascript and text/javascript mimetypes to
javascript.xml
application/javascript is the main one used in shared-mime-info,
without it syntax highlighting is not working for *.mjs files.
text/javascript is added as a safeguard in case if shared-mime-info
switches to it later, as there is ambiguity between which of those two
should be the main one, and IANA could switch to text/javascript.
Refs:
* https://cgit.freedesktop.org/xdg/shared-mime-info/tree/freedesktop.org.xml.in
* https://www.iana.org/assignments/media-types/application/javascript
* https://tools.ietf.org/html/rfc4329
* https://html.spec.whatwg.org/multipage/scripting.html#javascript-mime-type
* https://github.com/bmeck/I-D/blob/master/javascript-mjs/draft.md