Add support for terser-webpack-plugin (webpack@4.26.0+)

[pleunv]

Webpack 4.26.0 switched from using uglifyjs-webpack-plugin to terser-webpack-plugin. Since OfflinePlugin seems to partly piggyback off webpack's minifier but also contains uglifyjs-specific code this is now throwing up the following error:

OfflinePlugin: uglifyjs-webpack-plugin is required to preform a minification

Messed around a little bit myself but adding terser support while maintaining uglifyjs support for < webpack@4.26.0 involves a bit more than updating the import, unfortunately.

Those running into this issue can meanwhile explicitly add uglifyjs-webpack-plugin to the dev dependencies until this is resolved.

[GGAlanSmithee]

@pleunv thanks for the report 👍

[Bobgy]

I tried to patch offline-plugin in my forked branch about this issue. What I did was simply passing uglifyOptions as terserOptions and that fixed the issue for me because terser seems to only have new options over uglifyjs.

I will try to clean up the implementation and submit a PR.

[Bobgy]

@GGAlanSmithee I've submitted the PR. Any chance you can merge it?
It's passing CI, so I need someone for code review.

[GGAlanSmithee]

@Bobgy I've left a few comments, and I think we can merge this. However, only @NekR can actually deploy this to npm, so he will have the last word.