Add StorageDriverIndexedDb

[sgwilym]

What's the problem you solved?

This adds a very basic version of a storage driver that persists to IndexedDB. 5mb localstorage shackles be gone!

What solution are you recommending?

Upon the first usage of the storage, docs are hydrated into memory from a single IndexedDB row. Every time the storage upserts a doc, it writes all docs in memory back into that single row. I told you it was basic!

We do this because reading and writing stuff to IndexedDB is very slow. But this isn't an approach that'll last because of the aforementioned 'loads the whole space into memory' thing.

The storage config is stored inside of a separate object store, and works more or less as you would normally use IndexedDB.