Add stylesheet to plain text files in .htaccess

Add stylesheet to plain text files in .htaccess

It turns out that Firefox (version 140 at least) will happily use an
HTTP Link header for a CSS stylesheet on a plain text file.  It works
a bit weirdly because it applies the stylesheet to the HTML conversion
of the file, but that actually means that the existing bedstead.css
works pretty well.

This doesn't work on Chromium, but that's no worse than what we've
already got.