docs: Add VSCode macOS Downloads folder troubleshooting and version requirements

[Copilot]

Addresses issues with VSCode setup on macOS and clarifies minimum version requirements for the Uno Platform extension.

Changes Made

macOS Downloads Folder Issue

Added comprehensive troubleshooting documentation for a common macOS issue where VSCode left in the Downloads folder operates in read-only mode, preventing proper extension functionality and updates. The documentation now includes:

• Clear explanation of the problem and its symptoms
• Step-by-step solution to move VSCode to the Applications folder
• Prevention guidance for new users during initial setup

Version Requirements Documentation

Based on dependency analysis showing alignment with Microsoft's C# extension requirements, added explicit version requirements:

• Uno Platform extension: VSCode 1.75.0 or later
• C# extension (OmniSharp mode): VSCode 1.93.0 or later
• C# Dev Kit: VSCode 1.95.0 or later

New Error Code

Added UVSC0003 error code to the build error documentation to help users troubleshoot version-related issues, including macOS-specific guidance.

Files Modified

• doc/articles/common-issues-vscode.md - Added macOS troubleshooting section and version requirements
• doc/articles/get-started-vscode.md - Updated prerequisites and configuration steps
• doc/articles/uno-build-error-codes.md - Added new UVSC0003 error code

These changes provide users with clear, actionable solutions to common VSCode setup issues on macOS while ensuring they have the minimum required versions for proper Uno Platform extension functionality.

Fixes #16644

Fixes #20611

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