Move from OrderedMap to IndexMap for custom properties

[CYBAI]

After #22555 merged, we've introduced the indexmap crate.
There's a similar implementation in custom properies called OrderedMap.

servo/components/style/custom_properties.rs

Lines 143 to 153 in bffe2a6

	/// A map that preserves order for the keys, and that is easily indexable.
	#[derive(Clone, Debug, Eq, PartialEq)]
	pub struct OrderedMap<K, V>
	where
	K: PrecomputedHash + Hash + Eq + Clone,
	{
	/// Key index.
	index: Vec<K>,
	/// Key-value map.
	values: PrecomputedHashMap<K, V>,
	}

It would be great to reuse the indexmap crate and remove the OrderedMap instead.

cc @emilio

[shanavas786]

@highfive assign me

[highfive]

Hey @shanavas786! Thanks for your interest in working on this issue. It's now assigned to you!