Allow ${foo} syntax to be either escaped or optionally skip variable substitution

See discussion on AESH-324. If a project handles its own variable substitution, it's easy enough to disable Aesh's handling (enableExport). But in Artificer's case, we want both: 1.) parameter substitution within our own query syntax and 2.) Aesh variables.

For #1, spederse@redhat.com suggested that if the variable syntax appeared within quotes, Aesh shouldn't process it. I get the point, but I'm wondering if that's not the best route. I can see multiple situations where quoted text would still want Aesh's variable handling.

The answer might simply be that Artificer needs to use something other than ${foo} for substitution, which I'm ok with...