Set up code owners

[tillkruss]

As per @felixarntz suggestion in #2, should we set up a CODEOWNERS file to automatically assign issues and PRs?

It would help reduce the noise.

[JustinyAhin]

@tillkruss it would definitely make sense to set up that to automatically assign PR reviewers. Not sure why/if we should do the same for issues though.

[JustinyAhin]

And from the GitHub documentation you shared, it is only for PRs.

[felixarntz]

I just realized that the current CODEOWNERS file is not fully accurate:

• It still contains modules/webp-uploads but following Module Slugs Naming Proposal #46 all modules will be in focus area-specific directories, so this needs to be changed to modules/images - which actually makes the CODEOWNERS file maintenance a lot simpler in the long run.
• Based on the above, I suggest we now already add the code owners for all 5 focus areas, even if there's no code yet. We couldn't do this in the way it currently is since we don't know in advance which modules will exist, but we do already know the focus areas, so we can add the focus POCs as code owners, for modules/images, modules/javascript, modules/site-health, modules/measurement, and modules/object-caching.

I suggest we open a follow-up PR for this issue to fix/expand this.

cc @JustinyAhin @tillkruss