Add file-nesting patterns for C# development files

[Copilot]

This PR implements the requested file-nesting patterns to improve file organization in the VS Code Explorer for C# development projects.

What this adds

The extension now contributes default file-nesting patterns via configurationDefaults in package.json that automatically organize related files under their parent files when file nesting is enabled in VS Code.

File-nesting patterns included

• C# source files (*.cs) - nests generated and designer files:

  • Program.designer.cs → nested under Program.cs
  • Program.g.cs → nested under Program.cs
  • Program.generated.cs → nested under Program.cs

• Project files (*.csproj) - nests user-specific settings:

  • MyProject.csproj.user → nested under MyProject.csproj

• Configuration files (*.json) - nests environment-specific configs:

  • appsettings.Development.json → nested under appsettings.json

• Razor pages (*.cshtml) - nests code-behind and styles:

  • Index.cshtml.cs → nested under Index.cshtml
  • Index.cshtml.css → nested under Index.cshtml

• Blazor components (*.razor) - nests code-behind and styles:

  • Counter.razor.cs → nested under Counter.razor
  • Counter.razor.css → nested under Counter.razor

• XAML files (*.xaml) - nests code-behind:

  • MainWindow.xaml.cs → nested under MainWindow.xaml

How it works

The patterns use VS Code's built-in file nesting feature with the ${capture} token to match the base filename. When a user has explorer.fileNesting.enabled: true in their settings, these patterns will automatically organize their C# project files for a cleaner Explorer view.

Implementation details

• Created complete VS Code extension structure with proper TypeScript compilation
• Added configurationDefaults section to package.json with the exact patterns requested
• Updated documentation to describe the file organization features
• Validated all JSON syntax and extension structure

Fixes #1823.

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