5345: Semantic Highlighting: Emit mutable modifier for 'self' when applicable r=matklad a=Veykril
This PR implements emitting the mutable modifier for the self keyword when applicable for semantic highlighting as mentioned in #5041. The rendered highlighting test html file:
![firefox_5lbsFNBqsT](https://user-images.githubusercontent.com/3757771/87346245-a5a07280-c551-11ea-9051-e5901255f8c9.png)
As you can see it does not emit the modifier when `self` is not used in a mutable context even if it is declared mutably in the enclosing function. I'm not sure if this is actually something wanted or not.
Co-authored-by: Lukas Wirth <lukastw97@gmail.com>