removed metachar:sym<$( )>

Which was my first approach to support contextualizers.
Since this is not the way STD does it, and since it doesnt work right,
it gets reverted.
move <sigil> and <twigil>

... to HLL::Grammar since we are using it from NQP::Grammar
and Perl6::Grammar.
move $( ~ ) into variable token

This way contextualizers in code, quote and regexes do all the same,
and we can get rid of circumfix:sym<sigil>.