feat: CSS nesting baseline support

[rviscomi]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

Add support for evaluating baseline support for CSS nesting

What changes did you make? (Give an overview)

• added a test for CSS nesting
• added a handler for the NestingSelector key that checks for nesting selector availability
• added a check to isSupported so unknown feature statuses are supported by default (if the nesting ID happens to change, the test will fail and there won't be a runtime error)

Related Issues

fixes #85

Is there anything you'd like reviewers to focus on?

Should the nesting selector ID be saved as a constant somewhere, or is it ok to hard-code it in the NestingSelector handler?

[nzakas]

LGTM.

Should the nesting selector ID be saved as a constant somewhere, or is it ok to hard-code it in the NestingSelector handler?

I think it's fine to keep it local to where it's being used. 👍