feature-set is now web-features

Hi all,

Starting later today, the feature-set repository will be web-features.

As discussed in July, to reduce some confusion around the names of the
feature set repo and package, we're finally renaming web-platform-dx/
*feature-set* to web-platform-dx/*web-features*. This means that the repo
will have a name that matches the published package (web-features) on npm.

This ought to be minimally annoying, as GitHub will redirect existing links
to the repository and I've prepared PRs to point to the new name and URL
within the web-platform-dx org on GitHub and on MDN. That said, if you run
into issues, please @ me (ddbeck) on issues or PRs for this — I'll endeavor
to fix them promptly.

Thank you!

Daniel