Check every existing syntax in syntax_for_file.

In answer to https://github.com/grosser/ultraviolet/commit/ad1f2b06f5751517d69d31535113b3b1383c7467#commitcomment-3023013

Note I'm a beginner in Ruby (though I'm experimented with other programming languages), so this code might not work as I expected, but you hopefully get the idea.