Use a different namespace to reference local variables

[stasm]

@markusicu wrote in #230 (comment):

… once a variable has been subject to select-and-format, using it should require different syntax from regular variables, because we need to be clear that the placeholder is replaced consistently in each variant. If you just write {$count} then a developer has to wonder whether their formatting options apply; and you cannot allow formatting options on a select-and-format variable in the variant, because then the output may well not fit the selected variant any more (e.g., changing the number of fraction digits on a plural). So pick a symbol other than the dollar, and don't allow functions and options there.

[romulocintra]

The syntax on develop branch it's using the same sigil - We might open it again depending on user's feedback