Add gazelle support and examples

[yujunz]

Add gazelle language extension for CUE, which enables automatic generation of Bazel BUILD files for CUE projects

[yujunz]

@seh would you be interested for gazelle language extension for cue?

It was originally implemented by @tnarg and adapted by @lenage. We have been using it in production for years.

[seh]

would you be interested for gazelle language extension for cue?

Yes, I am definitely interested. I had reviewed @tnarg's extension several years ago when I was first forking his ruleset, and at that point I didn't see a way to achieve what I needed out of such an extension. I'll take a look at what you shared here, and probably have questions for you.