Allow setting custom storage engine per atom

[kytta]

Closes #27

This is my shot at implementing a custom engine per atom. My particular use case: using one store in localStorage and the other in sessionStorage.

[kytta]

Todos for this PR:

• update index.js
• update index.d.ts
• write tests
• size-limit fails; either increase the limit or try to compact the code

[ai]

size-limit fails; either increase the limit or try to compact the code

Update limit to track the current progress in diff.

[ai]

Looks good. Rebase to new main, write test, and we can merge it.

[kytta]

I have just noticed, that I forgot about persistentMap; I think we should add the custom storage there too, since it also uses the PersistentOptions type

[ai]

We need to test that didn’t change default storage engine

[ai]

I tried to merge it and fix persistentMap but I found that you're changing only storageEngine. But we need also to change eventEngine.

Maybe we need another API for that.