Allow user-defined AnimatedIconData

### Use case

Currently `_AnimatedIconData` is private, which means that if a Developer would like to define their own `AnimatedIcons` using the `AnimatedIcon` widget and `vitool`, they can't.

References:
- https://github.com/flutter/flutter/blob/a913d39131af39383f08f5ffcfa4710522581157/packages/flutter/lib/src/material/animated_icons/animated_icons_data.dart#L107

### Proposal

- Rename `_AnimatedIconData` to `AnimatedPathIconData` (or rename to `AnimatedIconData` and remove previous)
- Rename `_PathFrames` to `PathFrames`
- Rename `_PathCommand` to `PathCommand`
- Rename `_PathMoveTo` to `PathMoveTo`
- Rename `_PathCubicTo` to `PathCubicTo`
- Rename `_PathLineTo` to `PathLineTo`
- Rename `_PathClose` to `PathClose`
- Update the files at https://github.com/flutter/flutter/tree/a913d39131af39383f08f5ffcfa4710522581157/packages/flutter/lib/src/material/animated_icons/data to use the new renamed classes
- Update `vitool` to generate using the renamed classes