fix: Make IS_MAYBE_MIRAGE work in Firefox

- FF vs Chrome produces different `toString()` for native `window.fetch`.
- This patch fixes that.

packages/request/src/fetch.ts

@@ -40,7 +40,8 @@ if (DEBUG) {
     Boolean(
       typeof window !== 'undefined' &&
         ((window as { server?: { pretender: unknown } }).server?.pretender ||
-          window.fetch.toString() !== 'function fetch() { [native code] }')
+          window.fetch.toString() !== 'function fetch() { [native code] }') ||
+          window.fetch.toString() !== 'function fetch() {\n    [native code]\n}'
     );
 }