fix(storage): browser module resoltuion is not picked up by metro web

[HuiSF]

Description of changes

Issue #, if available

https://github.com/aws-amplify/amplify-js/issues/13853

Description of how you validated changes

When using the library with Expo, and run npx expo start, observed that the Storage uses fetch instead of xhr for making network requests. This caused that the Storage.get() returns a non Blob, response.Body.

The metro bundler respect the browser module in package.json, but observed it didn't pick up the resolution during testing. The cause is that the module entry doesn't contain an extension name.

Checklist

• [x] PR description included
• [ ] yarn test passes
• [ ] Unit Tests are changed or added
• [ ] Relevant documentation is changed or added (and PR referenced)

Checklist for repo maintainers

• [ ] Verify E2E tests for existing workflows are working as expected or add E2E tests for newly added workflows
• [ ] New source file paths included in this PR have been added to CODEOWNERS, if appropriate

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.