Add license flag to create license file when running ncc

[thboop]

This PR addresses #569

It adds the --license flag to use license-webpack-plugin to generate a license file alongside the ncc output to adhere to the terms of common software licenses.

Currently, there is an open issue where that plugin outputs some warnings, so the output for a run looks like

ncc: Version 0.22.3
ncc: Compiling file index.js
(node:75823) DeprecationWarning: Chunk.modulesIterable: Use new ChunkGraph API
(node:75823) DeprecationWarning: Chunk.entryModule: Use new ChunkGraph API
 63kB  dist/licenses.txt
811kB  dist/index.js

That is being tracked here and is hopefully resolved so we can minimize the output.

I did some local testing, but did not add any unit tests, let me know if there's some testing that would be worthwhile to add, and if you could provide some guidance on how to do so, I wasn't able to find a contributing.md or documentation.

[thboop]

this was just moved above so we can dynamically add the LicenseWebpackPlugin to this array based on the results of the license flag

[styfle]

Perfect, thanks! 🥇

[styfle]

Currently, there is an open issue where that plugin outputs some warnings

Looks like the deprecation warnings were fixed in 2.3.3

xz64/license-webpack-plugin#67 (comment)