Add ability to inject functions into the workers

[DeMoorJasper]

Changes the way workers are used from registering functions/file to just requesting a function with it's path. Using mkHandle

Example:

let something = workerfarm.mkhandle(require.resolve('./worker.js'), 'doSomething');
let result = await something(...args);

[codecov-io]

Codecov Report

Merging #1235 into master will increase coverage by 2.75%.
The diff coverage is 65%.

@@            Coverage Diff             @@
##           master    #1235      +/-   ##
==========================================
+ Coverage   85.86%   88.61%   +2.75%     
==========================================
  Files          77       78       +1     
  Lines        4307     4384      +77     
==========================================
+ Hits         3698     3885     +187     
+ Misses        609      499     -110

Impacted Files	Coverage Δ
src/workerfarm/Worker.js	68.42% <0%> (-0.33%)	⬇️
src/workerfarm/child.js	95.12% <100%> (+0.03%)	⬆️
src/workerfarm/childBootstrap.js	100% <100%> (ø)
src/workerfarm/WorkerFarm.js	46.66% <42.85%> (-2.69%)	⬇️
src/assets/VueAsset.js	80% <0%> (-6.67%)	⬇️
src/visitors/dependencies.js	89.13% <0%> (-5.44%)	⬇️
src/assets/GLSLAsset.js	93.1% <0%> (-3.2%)	⬇️
src/transforms/uglify.js	71.73% <0%> (-2.18%)	⬇️
src/utils/serializeObject.js	88.23% <0%> (-1.77%)	⬇️
... and 21 more

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[devongovett]

Most plugins are asset types and already run in workers right?

[devongovett]

Maybe we could autodetect these instead of needing to list them? Basically Object.keys(require(workerPath))?

[DeMoorJasper]

I can change it, but think it'll be more precise if it has to be defined when being called.
For example if a worker has more than just the worker functions as exports we don't add a handler for all of them.
If we document this eventually it should be fine to use export keys I guess.

[DeMoorJasper]

@devongovett Yes that's true, the typescript one utilises workers for speeding up error handling. It would be faster and probably better if the plugin wouldn't have to start and handle it's own workers.
Not sure how this exactly works I haven't dug into the code of parcel-plugin-ts

[devongovett]

Also seems like this would potentially lead to different modules overwriting each other if they had the same function names. What if instead we just made the processRequest of the WorkerFarm work on both ends. So when you made a call to the worker farm, you'd specify both a module path and a function name rather than only having one module loaded into the children. This would work exactly the same way as when we call from children to parents. What do you think?

[DeMoorJasper]

@devongovett If this doesn't impact performance or code readability it's probably a great idea. Would definitely solve the problem of 2 plugins possibly having a naming conflict