eina: update vpath to also support a more classic syntax for variable.

This enable vpath to recognize also ${} as a variable. It does mimic
what Efl.ViewModel provide with Efl.ViewModel.PropertyText and various
other language.

Reviewed-by: Marcel Hollerbach <mail@marcel-hollerbach.de>
Differential Revision: https://phab.enlightenment.org/D8883