Fixed Singer issue

[mohitvdx]

PR order before #383

This is a Follow up PR from the discussion in #383.
It solves the Singer export and import issues.

Kindly review @walterbender

[mohitvdx]

import { _state, noteValueToSeconds, _defaultSynth, _polySynth } from '@/singer';

we're importing _polySynth from the @singer file, but it's not defined there

function _getSynth(synthType: string) {
  switch (synthType) {
    case 'polysynth':
      return _polySynth;
  }
  return _defaultSynth;
}

and then we're returning it in this switch statement.
_polySynth is not defined anywhere in the project files.

How should I proceed? @walterbender

[walterbender]

polysynth should be defined as one of the builtin synths in core.synthutils