feat: effect::immediate::batch

[QuartzLibrary]

This PR introduces a batch function that was present in an older version of leptos. It was removed because the new effects are deferred, so they are ~always batched.

Why is batch useful?
When using ImmediateEffects, you might want to modify multiple signals atomically to ensure all subscribers always see a consistent view.
This can come up because of application invariants that are hard or inconvenient to encode in the type system, to avoid excessive updates, or under certain design patterns like applying multiple edit operations to a signal tree.

In general, normal effects should still be preferred.

[gbj]

Awesome, thanks for reintroducing this and for the test.