SI-7848 Xlint no warn on $sym with params

This idea brought to you by retronym.

Also improve implicitNotFound detection at typer;
and avoid checking the standard interpolation
expression for cases like s"some $$x".

Some minor refactorings of implicitNotFound strings.

The intersobralator allows extra spaces, i.e., trims.

(This merge faked me out somehow.  Here's to workflow. Looks like the earlier
commits were unchanged.)