JS copied with copyFiles still minified

[mattsbennett]

Recently we discovered (due to a related bug) we were re-minifying pre-minified JS libraries we were copying into place & content-hash-naming with copyFiles. This was unexpected due to the following copyFiles documentation:

webpack-encore/index.js

Lines 569 to 571 in c673202

	* Notes:
	* * No transformation is applied to the copied files (for instance
	* copying a CSS file won't minify it)

It probably shouldn't have been unexpected, given the copied files were JS and the terser plugin just processes everything in the build target by default, but these details are also somewhat "below the surface" when using an implementation like encore.

Perhaps some documentation could be added here for this use-case, highlighting that any pre-minified JS processed this way (e.g. third-party libraries) must also be explicitly excluded from repeat-minification with configureTerserPlugin?