add `Center` Layout widget support, - `margin: auto` for web

[softmarshmallow]

We can alternatively use Center widget for Flex centered widget, which can optionally be interpreted as margin: auto in css.

• Add design cases

References

• https://www.w3schools.com/css/tryit.asp?filename=trycss_margin_auto
• https://api.flutter.dev/flutter/widgets/Center-class.html