[resolvers/webpack] [New] add support for webpack 5 'externals function'

[jet2jet]

This PR allows functions for resolving externals for both webpack 5 and prior to webpack 5 (issue: #1966 )

• To handle configs for both webpack 5 and priors, this PR checks externals.length as webpack does.
• In webpack 5.15, getResolve function is added to the object in the first argument, so this PR provides a very simple getResolve implementation (using the return value of getResolveSync).
• In this PR async 'external functions' are still not supported (as async webpack config not supported; related: Add warning about async Webpack configs #1962)
  • synchronized-promise might be usable to resolve this, but I haven't tried yet. (If it is OK, I'll try to add using synchronized-promise into this PR or another.)

[coveralls]

Coverage increased (+0.3%) to 69.827% when pulling b7bce61 on jet2jet:support-webpack5-externals-function into e871a9a on benmosher:master.

[coveralls]

Coverage increased (+0.3%) to 69.827% when pulling b7bce61 on jet2jet:support-webpack5-externals-function into e871a9a on benmosher:master.

[coveralls]

Coverage increased (+11.3%) to 82.45% when pulling da647d7 on jet2jet:support-webpack5-externals-function into 7974acc on benmosher:master.

[ljharb]

nice work!