Add dotnet.config template to dotnet new

[Copilot]

Adds a new dotnet.config template to dotnet new to support Microsoft.Testing.Platform configuration and future dotnet CLI configuration needs.

Usage

# Create empty dotnet.config file (future-proofing for additional config options)
dotnet new dotnetconfig

# Create dotnet.config with test runner configuration
dotnet new dotnetconfig --runner "Microsoft.Testing.Platform"

# Alternative short name
dotnet new dotnet.config --runner "Microsoft.Testing.Platform"

Template Behavior

Empty by default: When no --runner parameter is specified, creates an empty file to allow future expansion for other dotnet.config settings.

With runner: When --runner is specified, generates proper INI format:

[dotnet.test.runner]
name = "Microsoft.Testing.Platform"

Implementation Details

• Template located in template_feed/Microsoft.DotNet.Common.ItemTemplates/content/DotnetConfig/
• Supports both dotnetconfig and dotnet.config short names
• Uses template engine conditional processing to include/exclude content based on parameters
• Follows existing template patterns similar to global.json and other config templates
• Includes proper localization support with auto-generated language files
• Help shows: -r, --runner <runner> The test runner to use.

This enables easy onboarding to Microsoft.Testing.Platform's new UX as requested, while providing a foundation for future dotnet.config functionality.

Fixes #47589.

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