Add copilot-setup-steps workflow for .NET SDK and Linux dependencies setup

[Copilot]

This PR adds a new GitHub Actions workflow copilot-setup-steps.yml that preinstalls the .NET SDKs and Linux dependencies required for BarcodeGenerator development and Copilot agent environments.

What this workflow does

The workflow sets up a complete development environment by:

• Installing multiple .NET SDK versions: 9.0.x, 8.0.x, and 6.0.x to support all target frameworks used by the project
• Installing Linux dependencies for SkiaSharp: libfontconfig1 and libfreetype6 packages needed for cross-platform graphics rendering
• Verifying the installation: Runs dotnet --info to confirm all SDKs are properly installed

Why this is needed

According to the repository's copilot-instructions.md:

• .NET SDK 9.x is required for full solution builds (test projects target .NET 9.0)
• .NET 8.x can build the main library individually
• .NET 6.0 is one of the target frameworks for the library
• SkiaSharp requires specific Linux native dependencies for proper operation

Workflow triggers

The workflow can be triggered by:

• Manual dispatch (workflow_dispatch)
• Changes to the workflow file itself (push/PR on the specific path)

This focused approach ensures the workflow only runs when needed for environment setup, aligning with its purpose of preparing development environments rather than performing builds or tests.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.