Improve primer/no-override to match classes, better reporting

[shawnbot]

This updates our primer/no-overrides rule to focus on class selectors specifically, as including non-class selectors produces a lot of false positives in (cough) some large custom CSS codebases. I've also reworked the error messages so that they're easier to read, and so that you can sort the output to more easily find what you're looking for:

#                                          ⇣ this is the full selector in violation
✖ ".text-gray" should not be overridden in ".foo .text-gray" (found in utilities)
# ⇡ this is the immutable part                      this is the bundle ⇡

[colebemis]

Looks good to me 👍 Left a couple comments but nothing blocking.