bevy_reflect: Add ReflectRef/ReflectMut/ReflectOwned convenience casting methods

[MrGVSV]

Objective

#13320 added convenience methods for casting a TypeInfo into its respective variant:

let info: &TypeInfo = <Vec<i32> as Typed>::type_info();

// We know `info` contains a `ListInfo`, so we can simply cast it:
let list_info: &ListInfo = info.as_list().unwrap();

This is especially helpful when you have already verified a type is a certain kind via ReflectRef, ReflectMut, ReflectOwned, or ReflectKind.

As mentioned in that PR, though, it would be useful to add similar convenience methods to those types as well.

Solution

Added convenience casting methods to ReflectRef, ReflectMut, and ReflectOwned.

With these methods, I was able to reduce our nesting in certain places throughout the crate.

Additionally, I took this opportunity to move these types (and ReflectKind) to their own module to help clean up the reflect module.

Testing

You can test locally by running:

cargo test --package bevy_reflect --all-features

---

Showcase

Convenience methods for casting ReflectRef, ReflectMut, and ReflectOwned into their respective variants has been added! This allows you to write cleaner code if you already know the kind of your reflected data:

// BEFORE
let ReflectRef::List(list) = list.reflect_ref() else {
    panic!("expected list");
};

// AFTER
let list = list.reflect_ref().as_list().unwrap();

[alice-i-cecile]

@MrGVSV if this is merge-conflict-free tomorrow for the merge train I'll get this in :)