Implement Reflect for indexmap::IndexMapand indexmap::IndexSet

[2ne1ugly]

Objective

• Fixes Reflect is not implemented for IndexMap/IndexSet #19681

Solution

• Implement Map, PartialReflect, Reflect, Typed, FromReflect, GetTypeRegistration for indexmap::IndexMap and indexmap::IndexSet
• Add #[derive(Reflect)] to EntityIndexSet
• Add indexmap feature to bevy_reflect and Update docs accordingly

Testing

• Added test on indexmap::IndexMap

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I initially tried using impl_reflect_for_hashmap, but ran into two issues:

1. IndexMap::drain requires a range parameter, unlike HashMap::drain.
2. IndexMap::remove defaults to swap_remove, and is now deprecated.

I went with shift_remove, since I think it matches user expectations.
That said, it’s O(n), while swap_remove is O(1).
Would love feedback on which behavior we should prefer.

[IronGremlin]

LGTM

[pablo-lua]

Looks pretty much straightforward and even comes with tests. LGTM

[alice-i-cecile]

@2ne1ugly I think your feature gates are slightly wrong: check the CI logs for details. I'd like to ship this in 0.18, but it's not a blocker, so I'm cutting it from the milestone and we'll add it in a later release candidate if it happens to get in.

[2ne1ugly]

Fixed the feature flag issue!