Use forward declares for vector math types

Adds operators to convert from int vector types to float vector types
as done in the upstream engine implementations.

include/godot_cpp/variant/vector3i.hpp

@@ -32,10 +32,12 @@
 #define GODOT_VECTOR3I_HPP
 
 #include <godot_cpp/core/math.hpp>
-#include <godot_cpp/variant/string.hpp>
 
 namespace godot {
 
+class String;
+class Vector3;
+
 class Vector3i {
 	_FORCE_INLINE_ GDNativeTypePtr _native_ptr() const { return (void *)this; }
 
@@ -107,6 +109,7 @@ public:
 	inline bool operator>=(const Vector3i &p_v) const;
 
 	operator String() const;
+	operator Vector3() const;
 
 	inline Vector3i() {}
 	inline Vector3i(int32_t p_x, int32_t p_y, int32_t p_z) {