Convert multiple selected files or folder within Visual Studio

[GrahamTheCoder]

Requested by Fredrik Olsson in a review: https://marketplace.visualstudio.com/items?itemName=SharpDevelopTeam.CodeConverter&ssr=false#review-details

There are some slight difficulties with getting this to work because the extension uses UIContextRules to control which menu items are available (before the extension loads). I'm not sure how to get them working for multi-selections or folders in just the right cases. However, we could take the approach of displaying the menu items all the time, and then just displaying a message box if no VB/C# is found in the selection when clicked. Very happy to accept a PR for this.