Added an init_bundle method to World

[13ros27]

Objective

Make it easy to get the ids of all the components in a bundle (and initialise any components not yet initialised). This is fairly similar to the Bundle::get_component_ids() method added in the observers PR however that will return none for any non-initialised components. This is exactly the API space covered by Bundle::component_ids() however that isn't possible to call outside of bevy_ecs as it requires &mut Components and &mut Storages.

Solution

Added World.init_bundle<B: Bundle>() which similarly to init_component and init_resource, initialises all components in the bundle and returns a vector of their component ids.

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Changelog

Added the method init_bundle to World as a counterpart to init_component and init_resource.

[james7132]

Suggested change

	self.bundles.get(id).unwrap().components()
	// SAFETY: The bundle was just initialized. The bundle must still exist.
	unsafe { self.bundles.get(id).debug_unwrap_unchecked().components() }

We should be avoid panicking in the ECS when we know the invariants are satisfied. Ideally init_info should return a &BundleInfo to avoid the need for this, but that's out of the scope of the PR.

[13ros27]

I've changed it to unwrap_unchecked, I couldn't find a debug_unwrap_unchecked anywhere?

[james7132]

DebugCheckedUnwrap is an extension trait we have inside bevy_ecs. It's identical but panics in debug mode if the Option isn't actually Some.