Tweak Snapshot type so it treats regular arrays and maps the same as ArraySchema and MapSchema

[FTWinston]

I stupidly thought I was working on a fork, and pushed this to main. I've reverted it now. Sorry!

This PR allows the Snapshot type to treat Map types the same way it treats MapSchema types. It makes it easier to write prop types for react components without having to reference colyseus schema types directly.

In a shared package, I have:

interface IPlayerState {
    weapons: IMap<string, IWeaponState>;
}

In my engine package, I have:

class PlayerState extends Schema implements IPlayerState {
    @type({ map: WeaponState }) weapons = new MapSchema<WeaponState>();
}

And in my frontend package, I have:

const InventoryDisplay(props: { player: Snapshot<IPlayerState> }) {
    ...
}

This pattern already works for ArraySchema and classes extending Schema, but the transformation of MapSchema<T> to Record<string, T> means this change is needed to work with MapSchema.