Allow to assign variables in Debug.Session using e.g. $x = ...

[toivoh]

E.g. the syntax

$x = f(5)

would evaluate f(5) in the current scope, and assign the value to the variable Debug.Session.x.

Need to keep a pure interpolated symbol such as $x in the AST, make it look like a symbol, but not a normal one, and to provide the proper setter to graft.
Consider: Should all interpolations be evaluated at the point of occurrence instead of beforehand? Probably, would be inconsistent otherwise.