I've recently downloaded and the iOS app and when trying to create a wikipedia account, I've gotten too many "username not available" errors after typing every field (especially Captcha is annoying) and tapping "Create". This frustrated me so I went to the web and tried to create an account: to my surprise, web UI checked username validity on text field return, which didn't require me to input all the fields like password.

So I've inspected the API required and implemented it in the iOS app. I couldn't find any official documentation on this API so I've just tested with 3 username types: taken, not available, and available. I'm opening this PR as a draft since I'm sure it needs some changes before being mergeable. I'm not sure the response struct I've implemented can handle all responses this API could return.

Volunteer PR in draft: https://github.com/wikimedia/wikipedia-ios/pull/4647