Fix fetching unshipped locales when there is no remote base

Choosing a language other than English or Chinese (offered in Settings and by the "DevTools is now available in …" infobar) makes the frontend fetch `./locales/<locale>.json`, which React Native never ships: only `bundled_devtools_locales` are packaged, and the rest are meant to come from Chrome's `remoteBase` CDN, which React Native never passes. The dev server logs `Error: ENOENT: no such file or directory, stat '…/core/i18n/locales/it.json'` on every launch, and the frontend only falls back to English once the failed request comes back.

Reject such a locale in `fetchAndRegisterLocaleData` before fetching, so `MainImpl` falls back to `en-US` straight away. Locales served from a remote base, and bundled ones, load as before.

Fixes react/react-native#52461

**Test plan**

Served `@react-native/debugger-frontend@0.88.0` through `@react-native/dev-middleware@0.88.0` (same `serve-static` setup as Metro) and loaded `rn_fusebox.html` in headless Chrome with `navigator.language` set to `it-IT`.

- Before, with the `language` setting `it`: 404 for `locales/it.json`, the server logs the `ENOENT` from the issue, and the frontend warns and renders in English. The same happens with 0.79.5.
- With this change applied to the built `core/i18n/i18n.js`, for `it` and `de`: neither file is requested, there is no `ENOENT`, the UI renders in English and the warning gives the new cause. With `zh` the UI still renders in Chinese (`zh.json` 200), and the default setting loads only `en-US.json`.
- `npx eslint front_end/core/i18n/i18nImpl.ts` passes, and a scoped `tsc` reports no errors in `front_end/core/i18n/`. The karma unit tests were not run: this checkout has no GN build or depot_tools.

Author: huntie

front_end/core/i18n/i18nImpl.ts

@@ -65,6 +65,9 @@ function getLocaleFetchUrl(locale: Intl.UnicodeBCP47LocaleIdentifier, location:
  */
 export async function fetchAndRegisterLocaleData(
     locale: Intl.UnicodeBCP47LocaleIdentifier, location = self.location.toString()): Promise<void> {
+  if (!BUNDLED_LOCALES.has(locale) && !Root.Runtime.getRemoteBase(location)?.version) {
+    throw new Error(`Locale '${locale}' is not bundled and there is no remote base to fetch it from`);
+  }
   const localeDataTextPromise = fetch(getLocaleFetchUrl(locale, location)).then(result => result.json());
   const timeoutPromise =
       new Promise<never>((_, reject) => window.setTimeout(() => reject(new Error('timed out fetching locale')), 5000));