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:

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>