In order to use a hook that will get user mention candidates that are resolved, we need to first extract the logic to compute user chat mention candidates outside of the class component. There are two steps to this:

1. Lift the selectionState out of the class component
2. Move the computation logic out of the class component

This diff just covers step 1, and the following diff will cover the next step.

Addresses https://linear.app/comm/issue/ENG-6112/update-native-chat-input-bar-to-make-it-easier-to-use-hooks

Ran the app and did some testing with @ mentioning to confirm there were no visibile regressions. Also did some logging to make sure the @ mention suggestions were the same as before