Add syntax highlighting for Slint files

Joshua Goinsrequested to merge
work/redstrate/slint-syntax-highlighting into master

This copies the syntax highlighting definition found upstream (https://github.com/slint-ui/slint/blob/master/editors/kate/slint.ksyntaxhighlighter.xml). Since it's GPL and also recieved explicit permission (https://github.com/slint-ui/slint/discussions/10154) we can include it here, before it had to be manually installed.

It is modified slightly from its original, namely reducing some spaces and combining a RegExpr so it passes our validation. I plan on upstreaming those fixes to them.

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