feat: add main and cross axis alignments

[ElectricCookie]

overflow_view_demo.webm

This PR introduces Main and Cross Axis Alignments to the overflow view as well as a paramter to expand the initial child.

This addresses #11 and serves the use case of a toolbar which has a title on the left and some actions on the right, which is a very common pattern in user interfaces.

I also took the liberty of adding the web platform to the example folder as web is the easiest to launch platform and there is no native aspect to this package. While doing so and adding the parameters I also refactored the code of the example a bit.

This PR is still a draft as I still need to

• implement the fixed layout variant
• check tests
• re-implement the avatar overview demo, which stacks the avatars
• adjust the implementation of the overflow builder to receive the final amount of overflowing widgets, this can currently be false if more than 1 widget gets pushed to the overflow due to the overflow widget not fitting.
• refactor and clean up the implementation. This was a rough implementation that still can be made a bit nicer
• document changes in readme and demo video