ci: add sourcelink

[jeanlrnt]

This pull request adds several build and packaging improvements to the MiniJwt.Core project file, mainly to enhance debugging, source linking, and CI integration.

Build and packaging enhancements:

• Enabled publishing of the repository URL and embedding of untracked sources to improve source link support in published packages.
• Configured the build to include symbols and generate symbol packages in the snupkg format for better debugging support.
• Added a condition to set ContinuousIntegrationBuild to true when running in GitHub Actions, ensuring proper build metadata during CI runs.

[Copilot]

Pull request overview

This pull request adds build and packaging configurations to enable SourceLink support and improve debugging experience for the MiniJwt.Core NuGet package. The changes configure the project to publish repository URLs, embed untracked sources, include symbol packages in .snupkg format, and ensure proper build metadata during GitHub Actions CI runs.

Key Changes:

• Enables SourceLink properties for better debugging support with embedded source code
• Configures symbol package generation in .snupkg format
• Sets ContinuousIntegrationBuild flag when running in GitHub Actions

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