Migrate Free Variable syntax from $$x to x? to ?x

[LPTK]

Syntax is already available; just need to remove uses of the old one, and possibly repurpose it.

[LPTK]

Some notes:

x? may not be a good syntax; together with a type annotation ir"(x? : Int) + 1", it may look to some people too close to C-like ternary operators. It also sometimes makes for ambiguous parses (eg: in ir"x?:Int" there is currently a dirty trick to go around the fact it is parsed as a use of operator ?:).

Syntax ?x seems nice, as in ir"(?x:Int)+1".

We also need a corresponding syntax for quasicode. The nice thing about the ?x syntax is that we could also have it in QC, as it desugars to ?.x (then just have in scope a ? value that extends Dynamic and defines selectDynamic as returning Nothing, so we can type-check the code).