Implement Reflect for indexmap::IndexMapand indexmap::IndexSet (#20734)

# Objective
- Fixes #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

---
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.

Author: 2ne1ugly

crates/bevy_ecs/Cargo.toml

@@ -91,6 +91,7 @@ hotpatching = ["dep:subsecond"]
 bevy_ptr = { path = "../bevy_ptr", version = "0.18.0-dev" }
 bevy_reflect = { path = "../bevy_reflect", version = "0.18.0-dev", features = [
   "smallvec",
+  "indexmap",
 ], default-features = false, optional = true }
 bevy_tasks = { path = "../bevy_tasks", version = "0.18.0-dev", default-features = false }
 bevy_utils = { path = "../bevy_utils", version = "0.18.0-dev", default-features = false }

crates/bevy_ecs/src/entity/index_set.rs

@@ -22,7 +22,11 @@ use super::{Entity, EntityHash, EntitySetIterator};
 
 use bevy_platform::prelude::Box;
 
+#[cfg(feature = "bevy_reflect")]
+use bevy_reflect::Reflect;
+
 /// An [`IndexSet`] pre-configured to use [`EntityHash`] hashing.
+#[cfg_attr(feature = "bevy_reflect", derive(Reflect))]
 #[cfg_attr(feature = "serialize", derive(serde::Deserialize, serde::Serialize))]
 #[derive(Debug, Clone, Default)]
 pub struct EntityIndexSet(pub(crate) IndexSet<Entity, EntityHash>);

crates/bevy_reflect/Cargo.toml

@@ -10,7 +10,7 @@ keywords = ["bevy"]
 rust-version = "1.85.0"
 
 [features]
-default = ["std", "smallvec", "debug", "auto_register_inventory"]
+default = ["std", "smallvec", "indexmap", "debug", "auto_register_inventory"]
 
 # Features
 
@@ -36,6 +36,9 @@ glam = ["dep:glam"]
 ## Adds reflection support to `hashbrown` types.
 hashbrown = ["dep:hashbrown"]
 
+## Adds reflection support to `indexmap` types.
+indexmap = ["dep:indexmap"]
+
 ## Adds reflection support to `petgraph` types.
 petgraph = ["dep:petgraph", "std"]
 
@@ -116,6 +119,7 @@ smallvec = { version = "1", default-features = false, optional = true }
 glam = { version = "0.30.7", default-features = false, features = [
   "serde",
 ], optional = true }
+indexmap = { version = "2.5.0", default-features = false, optional = true }
 petgraph = { version = "0.8", features = ["serde-1"], optional = true }
 smol_str = { version = "0.2.0", default-features = false, features = [
   "serde",