Migrate to github.com/tc39?

[erights]

At https://github.com/zenparsing/es-private-fields/issues/28 I write

Though I cannot find it right now, on WeakRefs https://github.com/tc39/proposal-weakrefs I remember the question came up about whether it is too early for us to have posted it on the github.com/tc39 site, vs waiting for some future stage before migrating. IIRC @allenwb commented that, since it is already under active discussion by the committee, it help ECMA's record keeping duties for the repo to already be there. As a result of this comment, we did not migrate WeakRefs off that site while waiting for stage promotion.

I suggest the same applies here.