Define obtain a storage key

[annevk]

My intuition from reading w3c/IndexedDB#334 and seeing whatwg/html#3054 is that we should have an "obtain a storage key" as standalone algorithm as well. It takes an environment and returns a storage key or failure.

Initially it would encompass the first three steps of https://storage.spec.whatwg.org/#obtain-a-storage-shelf, namely:

1. Let key be environment’s origin.
2. If key is an opaque origin, then return failure.
3. If the user has disabled storage, then return failure.
4. Return key.

@whatwg/storage agreed?