[TextField] General improvements umbrella

[newoga]

This is an umbrella issue for tracking general improvements to the TextField component in preparation for implementing #2416. I think taking this iterative approach gives us the best chance for getting there while maintaining backwards compatibility:

General Improvements:

• Make underline a separate internal TextFieldUnderline component (issue [TextField] Optional underline #2475, PR [TextField] Initial implementation of optional underline #2476, PR [TextField] Fixes underline custom styles not inherited by focusedUnderline. #2559)
• Make hint text a separate internal TextFieldHint component (PR [TextField] Separate TextFieldHint into separate internal component. #2567)
• Make floating label a separate internal TextFieldLabel component (PR [TextField] Extract floating label #2701 by @alitaheri)
• Implement helper text/error text as a separate component (issue [TextField] Implement helper text #2474)
• Reimplement mixins using other approaches (ie HOCs or utility functions) and convert to ES6 class
• Extract current TextField input functionality in a separate composable TextInput component (will be default implementation for backwards compatibility)
• Implement character counting functionality (possibly also through separate internal component)

I think once we get through these steps, we can start experimenting using this component with different underlying value types as described on #2416 and start experimenting with approaches to customizing value rendering (such as render value as text, chip, etc.)

[alitaheri]

@newoga These are fantastic ideas! I like them all 😁 Thanks 👍

[newoga]

@alitaheri Glad to hear! Hopefully I can knock a few out this week!

[alitaheri]

@newoga I'm tackling your 3rd item ( TextFieldLabel ). don't go for it for now.

[newoga]

@alitaheri Cool! Thanks for the heads up.

[nathanmarks]

Going to close this in favour of other issues.