Define permission storage mechanism

[reillyeon]

From #105 @domenic writes:

This implies a storage mechanism, which raises a lot of questions. E.g., how long is this storage retained? Does it get cleared when cookies are cleared? Is it scoped to origin, to site, to Window-plus-workers-directly-or-indirectly-owned-by-that-Window, ...?

Probably this would benefit from some explicit specification of the storage mechanism.

WebUSB and Web Bluetooth do this through integration with [[PERMISSIONS]].