Opt out react-forget-runtime from being Forgotten

This caused a build error when Forget was used in an Expo app as the 
react-forget-runtime package was itself being compiled with Forget. This broke 
Metro as metro serializes modules to iifes, but the import syntax that was 
injected by the useMemoCachePolyfill flag was left behind 

In practice I don't think the runtime package needs to ever be compiled by 
Forget, so this PR opts out the whole file. This would also prevent builds from 
breaking if someone decided to use the "all" compilation mode. 

Test plan: Ran the expo app and verified that it now builds with no errors

compiler/packages/react-forget-runtime/src/index.ts

@@ -5,6 +5,8 @@
  * LICENSE file in the root directory of this source tree.
  */
 
+"use no forget";
+
 import invariant from "invariant";
 import * as React from "react";
 
@@ -24,7 +26,6 @@ const $empty = Symbol.for("react.memo_cache_sentinel");
  * DANGER: this hook is NEVER meant to be called directly!
  **/
 export function unstable_useMemoCache(size: number) {
-  "use no forget";
   return React.useState(() => {
     const $ = new Array(size);
     for (let ii = 0; ii < size; ii++) {