Proposal promotion: Anchors and Hit-Tests

Based on discussion on the CG calls and GitHub issues, the following
proposals have been promoted to their own repos:

Anchors <https://github.com/immersive-web/anchors>
Hit-Test <https://github.com/immersive-web/hit-test>

According to the proposals process
<https://github.com/immersive-web/proposals/blob/master/README.md> these
repos are now labeled as being in incubation. Further discussion or issues
regarding these proposals should take place on the new repositories.

Thanks!

--Brandon