use ini lexer for systemd service files

[Akuli]

A foo.service specific lexer could highlight valid and invalid keys differently, but this is much better than no syntax highlighting.

[birkenfeld]

Makes sense. Do you also want to add the other systemd related filenames (.socket, .target, .slice, and so on)?

As you can see from CI, you need to run make mapfiles afterwards to regenerate the mapping of patterns to lexers.

[Akuli]

I'm a bit unsure about *.path and *.swap. They seem quite generic. Someone might use foo.path to denote a file that contains a path, or bar.swap for a swap file.

[Anteru]

It seems unlikely that someone will try to syntax highlight a .swap swap file though, so I don't have concerns. And there's no programming language using .path as the file ending, is there?

[Akuli]

Good points. I don't think this will be a problem in practice.

[Anteru]

Merged, thanks!