This commit extends the range of mimetypes detected in `html.kak` to the
following:
* text/html
* text/x-html
* text/xml
* application/xml
* application/…+xml (e.g. xhtml, rss)
Static .xml file will also be highlighted as HTML.
Initialising the `comment_line_chars` and `comment_selection_chars` variables
in language support scripts created a hard dependency of those scripts
to `commenting.kak`, which would create errors when this script was not
loaded, e.g. when running tests.