This PR improves the types for the Store.
- renames `StoreSnapshot` to `SerializedStore`, which is the return type
of `Store.serialize`
- creates `StoreSnapshot` as a type for the return type of
`Store.getSnapshot` / the argument type for `Store.loadSnapshot`
- creates `TLStoreSnapshot` as the type used for the `TLStore`.
This came out of a session I had with a user. This should prevent
needing to import types from `@tldraw/store` directly.
### Change Type
- [x] `major` — Breaking change
### Test Plan
- [x] Unit Tests
### Release Notes
- [dev] Rename `StoreSnapshot` to `SerializedStore`
- [dev] Create new `StoreSnapshot` as type related to
`getSnapshot`/`loadSnapshot`
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