feat(webpack-plugin): webpack 5 configuration factory

[erickzhao]

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• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
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Summarize your changes:

Another take at #1556 for Webpack 5

[codecov]

Codecov Report

Merging #2776 (a3613e6) into master (9026eae) will increase coverage by 0.09%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master    #2776      +/-   ##
==========================================
+ Coverage   73.93%   74.03%   +0.09%     
==========================================
  Files          77       78       +1     
  Lines        2356     2365       +9     
  Branches      436      437       +1     
==========================================
+ Hits         1742     1751       +9     
  Misses        464      464              
  Partials      150      150              

Impacted Files	Coverage Δ
packages/plugin/webpack/src/WebpackPlugin.ts	37.94% <0.00%> (ø)
packages/plugin/webpack/src/WebpackConfig.ts	98.76% <100.00%> (+0.03%)	⬆️
packages/plugin/webpack/src/util/processConfig.ts	100.00% <100.00%> (ø)

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[nornagon]

It would be nice if this were ESM compatible, so that the config file here could be an ESM module (potentially w/ top-level await). I think @MarshallOfSound mentioned a module that will do the work of figuring out whether a file is CommonJS or ESM and "just do the right thing" i.e. require() or import()? Or maybe just import() is enough here?

[VerteDinde]

Tested this out locally, and it seems to be working well. I agree with nornagon above that the ESM compatibility would be nice to have, but if we want to merge this now and start working with it, we can make a tracking issue for that feature 👍