Flutter Web support

[CoderSpinoza]

Checklist

There are several steps necessary to enable web support for Kakao Flutter SDK.

• [x] Distinguish KA header for javascript environment
• [x] Authorize with window.open() (default method)
• [ ] Provide a way to authorize with location.href
• [ ] Allow CORS for Kakao API Server
• [ ] Release with web support

Login via KakaoTalk will be currently unavailable for web since it requires access token polling and other more complex mechanisms.

[github-actions[bot]]

Stale issue message

[wurikiji]

@CoderSpinoza Hi, how is it going? Are you still working on web support or not?