See https://github.com/flutter/flutter/issues/11168

* Add a text formatter interface used by EditingText. Provide some default implementations.

* self nits

* Handle -1 selection offsets

* review notes

* simplify regular expression

* Add whitelisting formatters. Use a custom phone number formatter in text demo.

* review notes

* not being able to addAll(null) is pretty annoying

* review notes

* partial tests

* Add tests

* didn’t end up needing mockito

* move to services

TextFormField doesn't actually have many properties because it wraps a
TextField. Rather than trying to document all the arguments in the
constructor, we now refer to the docs for TextField, which explain the
parameters in a more readable way.

Fixes #7245

After this patch, there are three major text input widgets:

 * EditableText. This widget is a low-level editing control that
   interacts with the IME and displays a blinking cursor.

 * TextField. This widget is a Material Design text field, with all the
   bells and whistles. It is highly configurable and can be reduced down
   to a fairly simple control by setting its `decoration` property to
   null.

 * TextFormField. This widget is a FormField that wraps a TextField.

This patch also replaces the InputValue data model for these widgets
with a Listenable TextEditingController, which is much more flexible.

Fixes #7031