Rearchitect of executable

[XavierChanth]

- What I did

• Rearchitected the package to remove a dependency on the flutter sdk

- How I did it

• Rebuilt my own base commands, and used pub packages for the base abstractions
• Moved the flutter template and flutter libraries to at_app_flutter (Will also open a PR with at_widgets)

- How to verify it

I tested thoroughly on Windows/MacOS/Linux, the executable is more reliable that before on all three platforms.

One issue still remains on Windows due this issue with pub: dart-lang/pub#2934
This causes every command to be run twice, however it is out of our control.
The executable will still work for any command, even if run twice.

flutter pub global deactivate at_app
flutter pub global activate at_app
at_app create path/to/test/project

- Description for the changelog
Rearchitected at_app for better maintainability and quality of life.