Support for Azure Functions development

[filipw]

Based on dotnet/vscode-csharp#1241

This can be spun off from the existing CSX support with some extra features, specific to Azure Functions:

• predefined set of implicit namespaces
• predefined set of DLLs available as if they were GACed (even though they aren't)
• support for project.json for external references (this is already there, however for .NET Core only, but functions are net46 only)

The biggest challenge would be to decide when a CSX is a "standard" one and when it is an Azure Function one.
At the very worst, if we can't infer it, we could have a mode switch in omnisharp.json.

[filipw]

Long story short, the key would be to incorporate https://github.com/Azure/azure-webjobs-sdk-script/blob/master/src/WebJobs.Script/Description/DotNet/FunctionMetadataResolver.cs and most other things should just fall in place.