Allow variables to be exported (and deprecate !Play)

[slurmulon]

Right now the only "export" construct we have is !Play, but it's highly limited as there can only be a single definition.

We should replace !Play with simply !, which can then be applied to any variable and can be made available in a separate section of bach.json (such as refs).

The new ! keyword could then be paired with ! default <Element> in order to completely eliminate the need for !Play.