Add different corner styles on Shapes

[rydmike]

Add features that enables using other corner shape than the default circular.

• ContinuousRectangleBorder
• Custom Squircle (super ellipses)
• Beveled

Also Stadium version of Squircle

The Squircle version will be a custom version while waiting for official Flutter support. If/when Flutter supports it, it will be changed to the official version.

It should be possible to have one common shape type for the default theme and another one as an adaptive response.

[rydmike]

This squircle study is step on the road to adding this:

https://github.com/rydmike/squircle_study

This issue flutter/flutter#91523 in Flutter repo is being used to track implementation of Squircle in the Flutter SDK. I might hold off adding Squircle support to FlexColorScheme until the shape is supported by Flutter SDK. Adding a custom squirlce shape to do it, would be an interim solution. Still let's see how long it takes before there is any progress on this in the Fluter SDK.