makes boxity of fast track macros configurable

Previously, all built-in macros were assumed to be whitebox, but that’s
actually not the case. Just quasiquote macros have to be whitebox, while
the rest can be blackbox.

This also fixes SI-8091, because blackbox macros are typechecked differently
and therefore the necessary implicit conversion kicks in. If f”...” were
to remain a whitebox macro, then due to the changes introduced in commit
https://github.com/scala/scala/commit/a3b33419b02cafb7e2c6fed6dd96151859fc7d77
we would have to explicitly ascribe its expansion as String to achieve the same effect.