Fix high memory usage (#173)

[bew]

Fixes #173

The toJson() call can use a lot of memory by generating a lot of stats information about modules.
But we don't use this information for this plugin, so we can only enable the generation of assets related data used for this plugin.

Read #173 for more information

[bew]

Based on webdeveric/webpack-assets-manifest#47 I simplified the implementation to be future-proof (effectively disabling everything by default instead of manually listing every field we don't want one by one).

[Kenneth-KT]

Not sure if the author still want to maintain this library.

If not, there is another plugin webpack-assets-manifest that do exactly the same thing and they have this problem fixed in this commit already, but they haven't released it yet.

Run yarn add --dev https://github.com/webdeveric/webpack-assets-manifest.git\#051243e to add it to your repo.

[mastilver]

Hi @bew

Could you fix the tests please?