C#: Update of VS Code settings.

[michaelnebel]

In this PR we

• Add the C# Dev Kit as a recommended VS Code plugin and set the C# Extractor solution as the default solution in the settings.
• Add the C# Format Usings as a recommend VS Code plugin and set some default sorting rules in the settings.
• Remove Tasks.json as the C# Dev Kit now has a built in build command.

[hvitved]

Thanks!