Move the node createWorker() method to the NodeMixin.

[dpvc]

This PR moves the node-based createWorker() function from the liteAdaptor into the generic NodeMixin that is used by the liteAdaptor as well as the jsdomAdaptor and the linkedomAdaptor, so that they can support speech generation in node as well.

The code is moved unchanged, but the liteBase class gets a stub for createWorker() that is overridden in the mix-in since one is required for the AbstractDOMAdaptor class that liteBase creates before using the mixin.

[codecov]

Codecov Report

❌ Patch coverage is 40.81633% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.72%. Comparing base (3167719) to head (71da86f).
⚠️ Report is 13 commits behind head on develop.

Files with missing lines	Patch %	Lines
ts/adaptors/NodeMixin.ts	36.36%	28 Missing ⚠️
ts/adaptors/liteAdaptor.ts	80.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1328      +/-   ##
===========================================
- Coverage    86.72%   86.72%   -0.01%     
===========================================
  Files          337      337              
  Lines        84131    84139       +8     
  Branches      3139     4768    +1629     
===========================================
+ Hits         72961    72967       +6     
+ Misses       11170    11149      -21     
- Partials         0       23      +23     

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[zorkow]

lgtm.