Add enableInternalCssProcessing option for webpack

[evoactivity]

When an app provides it's own webpack css pipeline there is no reason for the default webpack config to also process css files, you can end up with multiple versions of the same file output into the build and loaded by the JS chunks (eg. renaming the css files by configuring MiniCssExtractPlugin yourself).

I'm not sure how to add a test for this, some guidance here would be helpful. Do I add a new scenario? Do I just add a test to an existing scenario?

Some documentation should be added to explain this option.

[mansona]

I just noticed that you're targeting main here 🙈 main Webpack support doesn't work yet and it won't work this way any more, it will have its own webpack config when we eventually release it

[evoactivity]

Yeah, I switched to stable and there were 388 files in this PR 🙈. So I changed back until I had time to merge stable back in locally and push up a correct PR.

[evoactivity]

@mansona I've got this targeting the stable branch now.
What would be the best way to test this? I don't see any current tests for things like styleLoaderOptions, or cssLoaderOptions.

[NullVoxPopuli]

Is this still needed, given we have #2299 now?

[evoactivity]

I think so, you still end up with two rules processing css files when you use a custom pipeline, doubling the workload.

[mansona]

I was going to suggest that we try to test this but tbh I don't think it's worth putting the effort in for something like this. If you just add some sort of documentation for this somewhere (maybe the readme?) then we should be able to merge this 👍

[evoactivity]

I've added documentation to the readme, the failing tests I'm pretty sure are just flaky windows issues.

[evoactivity]

@mansona Anything else for me to do on this?