Deprecated context, added border property, updated example and readme.md

[chaudharydeepanshu]

I think these changes will help this package. Let me know if you have any questions about them.

Deprecated context -

• Change is non-breaking
• Use Layout builder to get parent widget constraints and use them for width and height if width and height are not provided.

Added border property -

• Change is non-breaking
• In case, when the border is given to two containers then the space fixer between them also supports the border is useful.

Updated the example -

• Shows the before and after effect of both horizontal and vertical space fixer in ListView.

Edit -

Added Ink based Space fixers -

• Change is non-breaking
• SpaceFixerVerticalInkLine , SpaceFixerHorizontalInkLine .

Updated the readme.md -

• Now Readme.md shows a Demo gif of the example.