Add Visual Studio extension for Linux platform support

VSIX extension that adds:
- "OpenMaui Linux App" project template in File → New → Project
- Pre-configured launch profiles for Linux debugging
- WSL integration for Windows developers
- x64 and ARM64 build configurations

Launch Profiles included:
- Linux (Local) - Direct execution
- Linux (WSL) - Run via WSL
- Linux (x64 Release) - Release build for x64
- Linux (ARM64 Release) - Release build for ARM64
- Publish Linux x64/ARM64 - Self-contained publishing

Build with: msbuild /p:Configuration=Release
Output: OpenMaui.VisualStudio.vsix

vsix/OpenMaui.VisualStudio/Resources/.gitkeep

@@ -0,0 +1,3 @@
+# Add the following images:
+# - Icon.png (128x128) - Extension icon
+# - Preview.png (200x200) - Preview image for marketplace