LibJS+LibWeb: Replace GlobalObject with Realm in object constructors

No functional changes - we can still very easily get to the global
object via `Realm::global_object()`. This is in preparation of moving
the intrinsics to the realm and no longer having to pass a global
object when allocating any object.
In a few (now, and many more in subsequent commits) places we get a
realm using `GlobalObject::associated_realm()`, this is intended to be
temporary. For example, create() functions will later receive the same
treatment and are passed a realm instead of a global object.

Userland/Libraries/LibJS/Runtime/BigIntConstructor.cpp

@@ -17,8 +17,8 @@ namespace JS {
 
 static const Crypto::SignedBigInteger BIGINT_ONE { 1 };
 
-BigIntConstructor::BigIntConstructor(GlobalObject& global_object)
-    : NativeFunction(vm().names.BigInt.as_string(), *global_object.function_prototype())
+BigIntConstructor::BigIntConstructor(Realm& realm)
+    : NativeFunction(vm().names.BigInt.as_string(), *realm.global_object().function_prototype())
 {
 }