Add semantic highlighting

[rillig]

Currently, R code looks pretty much unicolor since all identifiers are in the default color. Semantic Highlighting makes the code more colorful and easier to grasp.

I expect that JetBrains made this feature easy to adopt, so that you just have to tell some code which identifiers should take part in the highlighting, and the rest is done by the existing JetBrains code. I didn't try it out though.

[holgerbrandl]

I was never a friend of this feature, to be honest. But let's keep it open for discussion