Platform-specific extension

[shiqimei]

• documentation: https://code.visualstudio.com/api/working-with-extensions/publishing-extension#platformspecific-extensions
• sample repo: https://github.com/microsoft/vscode-platform-specific-sample
• rigrep-prebuilt: https://github.com/microsoft/ripgrep-prebuilt
• vscode-rigrep: https://github.com/microsoft/vscode-ripgrep