Improve highlighter

[JojOatXGME]

I am currently looking into improving the highlighter and maybe add some basic completions. I am still working on it, but whoever is interested can make some comments here.

Related issues:

• Add color settings #3
• Provide instructions how to use this plugin #52

[JojOatXGME]

Somehow, the highlighter is currently not reliable, at least not in very large files. I also have not yet found a good solution to test the highlighter. The test-solution from the documentation only seems to work for higher levels of severity, like for inspections. So it looks like I have to put some more research into this topic as anticipated.

[JojOatXGME]

The highlighting is still not reliable for very large files, but I haven't found the reason for it. I expect it is a bug somewhere in IntelliJ, but not sure. Anyway, I think it is time, so I will merge the changes this week.

This PR contains the first feature which performs basic code analysis on top of the AST. There is the risk that this update has a significant performance impact, but at least it performed well in my tests using some large files from nixpkgs.