feat: ✨ Multiple showcase at the same time (#113)

[Sahil-Simform]

Description

Multiple showcase at the same time. Added a parameter linkedShowcaseKeys in showcase which will
take list of keys to start those showcase at the same time

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Related Issues

Closes #113

[Sahil-Simform]

This implementation has some issues:
Widgets are racing to start the showcase, making it unpredictable which one will display its barrier first, leading to unexpected behavior.

Closing this PR due to this issue.

Steps to Reproduce:
Link the 1st, 2nd, and 3rd showcases and run the example application multiple times.