LibJS: Use u64 for the length parameter in Array::create()

This doesn't matter per se as the value is immediately validated to be
in the 0 to 2^32 - 1 range, but it avoids having to cast a number that
potentially doesn't fit into a size_t into one at the call site. More
often than not, array-like lengths are only validated to be <= 2^52 - 1,
i.e. MAX_SAFE_INTEGER.

This is fully backwards compatible with existing code as a size_t always
fits into an u64, but an u64 might not always fit into a size_t.

Userland/Libraries/LibJS/Runtime/Array.cpp

@@ -17,7 +17,7 @@
 namespace JS {
 
 // 10.4.2.2 ArrayCreate ( length [ , proto ] ), https://tc39.es/ecma262/#sec-arraycreate
-ThrowCompletionOr<Array*> Array::create(GlobalObject& global_object, size_t length, Object* prototype)
+ThrowCompletionOr<Array*> Array::create(GlobalObject& global_object, u64 length, Object* prototype)
 {
     auto& vm = global_object.vm();