TextInput component (and maybe others) don't work with useRef

[danoc]

The TextInput component has two uses of ref:

1. The public API accepts a ref prop
2. The component has an internal ref to programmatically focus the input element

The component attempts to merge the two refs but currently doesn't work if the consumer uses useRef.

This internal/external ref pattern is common for UI libraries and there are a few solutions:

• https://www.davedrinks.coffee/how-do-i-use-two-react-refs/
• https://github.com/gregberge/react-merge-refs

We should probably use one of these solutions rather than trying to do it ourselves. We should also audit the existing components that expose ref to see if they also don't work with useRef.