Updated to Godot HashMap, HashSet, and Pair types

[rpgshooter]

Replace std::unordered_set/map with Godot containers:

Addition to #880

Summary

Replace std::unordered_map and std::unordered_set with Godot's HashMap and HashSet in Terrain3DInstancer
Remove custom hash structs that are no longer needed (Godot's HashMapHasherDefault provides built-in hashing for Vector2i, Vector3, and Pair<F, S>)
This change prepares the codebase for C++20 compatibility

Changes

constants.h
Removed Vector2iHash, PairVector2iIntHash, and Vector3Hash structs
terrain_3d_instancer.h
Replaced <unordered_map> and <unordered_set> includes with Godot templates
Changed type aliases to use HashMap<Vector2i, Pair<RID, RID>> and HashSet<Pair<Vector2i, int>>
terrain_3d_instancer.cpp
Updated API calls: .empty() → .is_empty(), .count() → .has(), .emplace() → .insert()
Changed std::make_pair to Pair<Vector2i, int>
Updated iteration patterns to use KeyValue<K, V> for HashMap and explicit Pair access for HashSet

[rpgshooter]

Addendum, the constants.h build error due to not having a .cpp and potential build order would be resolved with this merge. Example of one of the build errors:
src/constants.h:1:1: note: 'std::hash' is defined in header ''; this is probably fixable by adding ""include functional""