Provide API that can set $-variables prior to entering the parser.

This can be useful in some scenarios:

• to define some cc numbers by the custom library user of sfizz (eg. _oncc$MY_KNOB_ID)
• to set the sample path variable externally. there are some ARIA sfz bundles that want such things
  for instance, see https://github.com/sfzinstruments/karoryfer.shinyguitar/blob/026d25a62135aeb1900d0720ae3f87aa80f107f2/Shinyguitar.bank.xml#L5