Add a UI control for text entry

[JonnyOThan]

Might be out of scope a bit, but I saw someone asking about how to do this. Creating new UI controls is tricky but possible.

Example from TweakScale Rescaled:

https://github.com/JonnyOThan/TweakScale/blob/dev/Source/UI_ScaleEditNumeric.cs

https://github.com/JonnyOThan/TweakScale/blob/4719b9ff96521463b8b31b8488c1c6e61b891845/Source/HarmonyPatching/Squad.cs#L99