Expression mutability annotation

[ethanrjs]

Add an annotation for expressions saying whether or not they are settable/mutable values. i.e, the lines of a sign are mutable but the players name is not.

I have repeatedly wasted time wondering whether or not I can change the value of an expression, and the documentation does not clarify whether or not I can. It would be a very nice time saving QoL feature :)

[AyhamAl-Ali]

Good suggestion 👍 it has already been suggested here #4157 and being worked on so expect to see it soon :D