feat: integrate StackOverflow into supported platforms list

[Dipti45sktech]

Summary

This PR adds support for Stack Overflow profiles to the shared PLATFORMS registry used for DevCard profile generation.

The update introduces a new stackoverflow platform configuration with the required URL pattern, metadata, and follow strategy so StackOverflow profiles can be handled consistently alongside existing supported platforms.

In addition, a dedicated test suite was added to validate:

• Proper registration of the StackOverflow platform
• Correct profile URL generation through getProfileUrl()
• No regressions to existing platform behavior

The shared package was also updated with standalone test support by adding vitest and a package-level test script, allowing tests to run independently within @devcard/shared.

StackOverflow Profile Format

StackOverflow profiles typically follow this structure:

{id}/{display-name}

Example:

1234/alex

To make this clearer for users, the platform configuration includes the placeholder:

e.g. 1234/username

Closes #25

[Dipti45sktech]

Hi @ShantKhatri , could you please review my PR whenever available.