…dj#969)

The Hakyll "check" command is used to validate links in your site's
output, but at the moment it only checks ".html" files. There is also
an "xhtml" extension associated with the application/xhtml+xml MIME
type that more or less contains HTML.

It's important for authors to use a different extension on xhtml+xml
files because browsers interpret them differently (they use a faster
but stricter parser). Thus simply renaming those files to "html" is
not recommended. Instead this commit teaches Hakyll to check "xhtml"
files as well.

A more thorough change would allow the user to specify what extensions
to consider as "html" -- for example, "htm" and "xhtm" are both
common. I however see little need for the complexity, since no one
else has hit the issue in so many years. This commit therefore makes
the smallest change possible and hard-codes "xhtml" as an additional
extension to be checked.

Notably, I have not added "index.xhtml" as a default document in
checkFileExists, because developing locally (file://) I'm forced to
avoid such things in the first place.