Move scan_uage to its own package

[VisruthSK]

Cleaner separation of concerns? Useful if any other packages want the AST parsing but don't need the Stan stuff. Maybe split later if someone asks.

Call it funscanr?

[VisruthSK]

Leaving issue up for now. Code is pretty orthogonal currently, so splitting the AST parsing so other packages wouldn't have to roll their own if they wanted to have a similar function to stan_cite() is pretty simple, but this seems like unecessary dev time until someone expresses interest.