chore: add a locales stub for correct vite module resolution

When resolving `@blocknote/core/locales` locally, vite can pick up on this file within the `src` directory and use that for the locales export.
This should be the same as what is exported in that sub-package anyway

Author: nperez0111

packages/core/src/locales.ts

@@ -0,0 +1 @@
+export * from "./i18n/index.js";