fix(#1041): respect output.cssFilename and output.cssChunkFilename

[SukkaW]

Summary

Fixes #1041 by implementing #1041 (comment)

What kind of change does this PR introduce?

Moving filename and chunkFilename normalization into apply (because only then do we have access to the compiler object).

Now the precedence is:

this.options.filename -> compiler.options.output.cssFilename -> DEFAULT_FILENAME
this.options.chunkFilename -> compiler.options.output.cssChunkFilename -> getChunkFilenameFromFilename(filename)

Did you add tests for your changes?

Not yet, need help on where to add the test case. I am not familiar with the test folder structure here.

Does this PR introduce a breaking change?

No, by always preferring passed in filename and chunkFilename the breaking change could be avoided.

If relevant, what needs to be documented once your changes are merged or what have you already documented?

Maybe mention that we will also respect output.cssFilename and output.cssChunkFilename in existing options docs.