chore!: migrate yaru.dart and yaru_icons.dart into yaru_widgets

[Feichtmeier]

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BREAKING CHANGE

This migrates yaru.dart and yaru_icons.dart into this repository, but all commits are kept 🤞

To merge this we need to check very precicly if everything is correctly merged

TODO

• [x] Check if the icons work correcty @Jupi007 (I moved your example to "YaruIcons" page in the example)
• [x] Check if the theme works correctly @Feichtmeier
• [x] Double check if I didn't make any mistake @spydon and/or @d-loose

Ref #835

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[Feichtmeier]

https://github.com/ubuntu/yaru_widgets.dart/assets/15329494/a84e100d-5a88-4e96-9317-402a97c94439

The goldens pass locally after clean/upgrade and regeneration So they will prbly need the nightly CI to pass again here, so you can ignore those failed golden tests

[Feichtmeier]

I think we can improve and simplify some files placements:

* `colors.dart` could be moved to the `foundation/` folder ;

* `icons_foundation/` could be merged with `foundation` ;

* `text/font/` should be moved to `assets/`, and `text/text_theme.dart` to `themes/` ;

* `icons_widgets/` to `widgets` ;

* and I'm not sure, but `theme_widgets/inherited_theme.dart` can maybe be moved to `widgets/` (or `themes/`) ?

Except that, this looks great. I didn't noticed any problem with icons :D

I kept the sub structure for this PR to make a first atempt where everything works. Maybe I could do this in a second PR? then I change from Fix to Ref in the PR desc And thank you very much for catching the font!

[Jupi007]

Maybe I could do this in a second PR?

Ofc, sounds good :+1: