Add optional Xquik backend

[kriptoburak]

Summary

• Add opt-in backend: xquik support for posting through a connected Xquik account.
• Keep the existing x-api backend as the default and enforce its credentials at runtime.
• Document the new inputs and examples, and rebuild the bundled action output.

Validation

• npm test
• npm run build
• git diff --check

[kriptoburak]

If this is useful and you merge it after checks pass, feel free to share the PR on X/Twitter and mention my handle, burakbayir. I can repost or retweet it to my 24k followers of developers and power users, which may help more relevant users discover this repo.