Add APPLY action for list(TRANSFORM)

Add a new APPLY action to list(TRANSFORM) that invokes a user-defined
function or macro for each element of the list, enabling arbitrary
per-element transformations.

The callable receives the current element value and an output variable
name, and must set the output variable in PARENT_SCOPE to the
transformed value.

Includes documentation, release notes, and tests for both function
and macro callables, OUTPUT_VARIABLE, error cases, and selector
combinations (AT, FOR, REGEX).
Refs: #27761