Add syntax highlighting for KDE/Kate

[larsgw]

I would like, at some point, to add syntax highlighting for KDE and the Kate editor. They use a custom XML-based format, so I doubt we can easily reuse existing syntax highlighters, though I don't exactly know which of those there are already.

Documentation: https://docs.kde.org/stable5/en/kate/katepart/highlight.html
Example syntax definition for TOML: https://invent.kde.org/frameworks/syntax-highlighting/-/blob/master/data/syntax/toml.xml