Add an APPLY action to $<LIST:TRANSFORM> that evaluates an arbitrary <body>
once per selected element, with $<_0> bound to the element, so a list can be
mapped through any generator expression at generate time. Unlike the
configure-time list(TRANSFORM ... APPLY <function>) command, the genex form has
no side effects and returns the body's value directly, and a list-valued result
expands into multiple elements.
The body evaluates in its own binding scope, so nested APPLY actions can shadow
$<_0>, and context-sensitive state it observes (such as target dependencies)
still propagates to the enclosing expression. APPLY accepts the same
AT/FOR/REGEX selectors as the canned actions.
Issue: #27892
