LibJS: Put Bytecode::CallFrame + register slots in a single allocation

The number of registers in a call frame never changes, so we can
allocate it at the end of the CallFrame object and save ourselves the
cost of allocating separate Vector storage for every call frame.

Userland/Libraries/LibJS/SourceTextModule.cpp

@@ -708,7 +708,7 @@ ThrowCompletionOr<void> SourceTextModule::execute_module(VM& vm, GCPtr<PromiseCa
                 result = value_and_frame.value.release_error();
             } else {
                 // Resulting value is in the accumulator.
-                result = value_and_frame.frame->registers.at(0).value_or(js_undefined());
+                result = value_and_frame.frame->registers()[0].value_or(js_undefined());
             }
         }