Add ViewStore._Snapshot

[tgrapperon]

extension ViewStore {
  /// This type provides access to the state of a store at the time this value is created.
  ///
  /// It is not intended for general use, and you should use a ``ViewStore`` if you want to
  /// access the state of a ``Store``.
  public struct _Snapshot {
    /// The ``Store``'s state at the time the ``ViewStore/Snapshot`` was created.
    public let state: State
    /// Creates a snapshot of the state of `store`.
    public init(store: Store<State, Action>) {
      self.state = store.state.value
    }
  }
}

While developing high-performance utilities as a third party, I often find myself limited by the need to create whole ViewStore to simply access the state of a Store at some point.

I'm understanding why the state of Store is internal, but I feel that this approach would put it far enough to avoid misuses.

As an aside, this would probably allow to extract SwiftUI and UIKit utilities into their own modules/libraries, which would likely help on the backward compatibility front (at the cost of an higher complexity for maintainers I admit). It would also make sense in the future, if TCA happens to not rely on Combine anymore.

This is only a proposal to get an efficient access to the Store's state as a third party. I'm open to any alternative, as long as I don't create a complete ViewStore.
What do you think about it?