Added instructions for VSIX Toolkit usage

[madskristensen]

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• My contribution adds a new instruction, prompt, or chat mode file in the correct directory.
• The file follows the required naming convention.
• The content is clearly structured and follows the example format.
• I have tested my instructions, prompt, or chat mode with GitHub Copilot.
• I have run npm start and verified that README.md is up to date.

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Description

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Type of Contribution

• New instruction file.
• New prompt file.
• New chat mode file.
• New collection file.
• Update to existing instruction, prompt, chat mode, or collection.
• Other (please specify):

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Additional Notes

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By submitting this pull request, I confirm that my contribution abides by the Code of Conduct and will be licensed under the MIT License.

[Copilot]

Pull request overview

This pull request adds comprehensive instructions for Visual Studio extension (VSIX) development using the Community.VisualStudio.Toolkit. The instructions provide detailed guidelines for creating VS extensions with proper threading patterns, theming support, and best practices.

Key Changes

• Added a new 647-line instruction file covering VSIX development with Community.VisualStudio.Toolkit
• Includes code examples for package classes, commands, options pages, MEF components, and UI theming
• Provides comprehensive threading guidelines and analyzer rules for safe VS extension development

[Copilot]

According to the custom coding guidelines for this repository, new instruction files should be added to the README. The PR checklist indicates that npm start has not been run to update the README.md. Please run npm start to regenerate the documentation and include the updated README files in this PR.