Handle overlapping squeeze

[Manishearth]

In https://github.com/immersive-web/webxr/issues/997 we decided that hand input profiles by default would be select-only, and created generic-hand-select.

We also discussed adding a way to opt in to overlapping squeeze behavior. There were two rough proposals:

• A feature named hand-overlapping-squeeze that enables a generic-hand-select-squeeze profile
• A generic-hand-overlappingsqueeze profile that signals squeeze events through a button that is not the "squeeze" button.

I have a slight preference for the former since the APIs for accessing the squeeze are the same. Thoughts?

[Manishearth]

If we do add a feature it's probably worth speccing it in the hand-input spec