This class looks like it _should_ give valid equality checks:

```dart
class _WidgetStateColorMapper extends _WidgetStateColorTransparent {
  const _WidgetStateColorMapper(this.map);

  final WidgetStateMap<Color> map;

  _WidgetStateMapper<Color> get _mapper => _WidgetStateMapper<Color>(map);

  @OverRide
  Color resolve(Set<WidgetState> states) => _mapper.resolve(states);

  @OverRide
  bool operator ==(Object other) {
    return other is _WidgetStateColorMapper && other.map == map;
  }

  @OverRide
  int get hashCode => map.hashCode;
}
```

But I made a mistake: I should have used `other._mapper == _mapper` and `_mapper.hashCode`.

<br>

It'd be pretty nice if no copy-pasting was needed in the first place:

```dart
class _WidgetStateColorMapper extends WidgetStateMapper<Color> implements WidgetStateColor {
  const _WidgetStateColorMapper(super.map);
}
```

<br><br>

This PR makes `WidgetStateMapper` a public class, similar to `WidgetStatePropertyAll`. The new API, when used incorrectly, will `throw` with a detailed error message, rather than silently returning irrelevant values.

**Changes**
- Add `WidgetStateInputBorder` class, with `.resolveWith()` and `.fromMap()` constructors
- Deprecate `MaterialStateOutlineInputBorder` and `MaterialStateUnderlineInputBorder` and provide data-driven fixes

<br>

**Other changes** based on #154972 (review)
- Fix documentation copy-paste typo ("OutlinedBorder" � "InputBorder")
- Add test to ensure borders are painted correctly
- Add DartPad sample & relevant test

**Changes**
- Add `WidgetStateInputBorder` class, with `.resolveWith()` and `.fromMap()` constructors
- Deprecate `MaterialStateOutlineInputBorder` and `MaterialStateUnderlineInputBorder` and provide data-driven fixes

<br>

**Other changes** based on flutter#154972 (review)
- Fix documentation copy-paste typo ("OutlinedBorder" � "InputBorder")
- Add test to ensure borders are painted correctly
- Add DartPad sample & relevant test