Move the Mascot from Demo2 into a new widget.
Make the Rat grey and adjust the other colors.
```rust
frame.render_widget(RatatuiMascot::default(), frame.area());
```
All the widgets now live in their own ratatui-widgets crate, but are re-exported in the main ratatui crate.
This makes it easier to use portions of the ratatui library and is part of the effort to modularize
Part of: #1388
---------
Co-authored-by: Orhun Parmaksız <orhun@archlinux.org>
Co-authored-by: Orhun Parmaksız <orhunparmaksiz@gmail.com>
