list(TRANSFORM): Add PREDICATE selector

Add a new `PREDICATE` selector to `list(TRANSFORM)` that uses a
user-defined function or macro to decide which elements to transform.

The predicate callable receives each element value and an output
variable name.  It must set the output variable to a truthy or falsy
value in PARENT_SCOPE; only elements for which the predicate returns
true are passed to the transform action.

Adds a shared PredicateEvaluator helper class used by both TRANSFORM
PREDICATE and (in a subsequent commit) FILTER PREDICATE.

Includes parser wiring, error and success tests covering all existing
actions (TOUPPER, TOLOWER, REPLACE, STRIP, GENEX_STRIP, APPEND,
PREPEND, APPLY) combined with the PREDICATE selector.

Issue: #27761