C# extension should have telemetry to record .NET Core vs. mono/desktop usage

[gregg-miskelly]

Currently the C# extension supports both desktop/mono and .NET Core scenarios. We should have telemetry so that we can understand which type of projects people are using.

[DustinCampbell]

This is going to take more work to bake the information we need into OmniSharp itself. Moving this out to the next release.