Do not highlight some types as keywords

[nicolasstucki]

Currently Unit, Boolean, Byte, Char, Short, Int, Float, Long, Double, String and Symbol are colored as keywords. When written down it look like there is an error in the code. We should highlight them as types instead for homogeneity.

[nicolasstucki]

For example in Symbol.scala#L33

the Symbol appears as a keyword. One could argue that this is highlighted as a scala.Symbol "primitive" but it is not as this symbol is defined that same file. It is impossible to distinguish them syntactically, only the language server could take the decision to display them with a specific style.

[martijnhoekstra]

if this is done, maybe self should stop being shown as a keyword too -- it's the reasoning of "not a keyword but sort of" as these I think.

[nicolasstucki]

Same for None and Nil