Instead of having a mere value, these would be able to compute a macro
expansion. We parse the contents inside the ${...} delimiters as a VAV,
but there can't be (yet?) nested curly {braces}, even quoted.

The first argument inside the delimiters is the macro name, and other
VAV arguments are treated as arguments to the macro's function.

For example ${foo,bar,baz} would call the a macro "foo"'s function with
arguments "bar" and "baz". Simple macros don't take arguments and work
as usual.