Che 7 CSharp support

[tsmaeder]

Make a Che 7 plugin for C#.

Since the VS Code extension for C# for Microsoft is licensed only for use with VS Code, we need to do what the Acute plugin for Eclipse did, namely create a Theia plugin using Samsung netcoredbg (https://github.com/Samsung/netcoredbg/releases) and Omnisharp-Roslyn (https://github.com/OmniSharp/omnisharp-roslyn).
Subtasks:

• Theia plugin: .Net Debugger Theia plugin: .Net Debugger #12790
• Theia plugin: C# Theia plugin: C# #12791
• Che 7 plugin: C# Che 7 plugin: C# #12792
• Che 7 plugin: .Net debugger Che 7 plugin: .Net debugger #12793

[tsmaeder]

We should look at the existing language plugin in order to find out what features it offers (commands, dialogs, etc. Anything that goes beyond what LSP offers).

[tsmaeder]

It makes no sense to make a vscode extension for this, because folks using VS Code will install the "official" c# support from microsoft. So Theia plugin it is!