Add RESX file nesting pattern for Designer.cs files

[Copilot]

This PR adds a file nesting pattern for RESX (Resource) files to automatically group auto-generated Designer.cs files under their corresponding .resx files in the VS Code file explorer.

Problem

In .NET projects, RESX resource files typically have associated auto-generated Designer.cs files that contain strongly-typed accessors for the resources. For example:

• Resources.resx has a corresponding Resources.Designer.cs
• Strings.resx has a corresponding Strings.Designer.cs

Currently, these files appear as separate items in the VS Code file explorer, making the project structure appear cluttered and less organized.

Solution

Added a new file nesting pattern to the explorer.fileNesting.patterns configuration:

"*.resx": "${capture}.Designer.cs"

This pattern follows the same convention used by other file types in the extension (e.g., *.xaml → ${capture}.xaml.cs).

Result

With this change, the VS Code file explorer will now automatically nest Designer.cs files under their parent RESX files:

📁 Properties/
  📄 Resources.resx
    └── 📄 Resources.Designer.cs
📁 Localization/
  📄 Strings.resx
    └── 📄 Strings.Designer.cs
  📄 ErrorMessages.resx
    └── 📄 ErrorMessages.Designer.cs

This provides cleaner file organization and reduces visual clutter in .NET projects that use resource files, making it easier for developers to navigate their project structure.

Fixes #[issue-number]

Fixes #8643

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[timheuer]

lgtm