Notify User on Linux, when only Unity mono is present

On Windows we ship MSBuild.
On Mac we display a dialog to install mono with `MonoInstallTrigger`.
On Linux we don't do anything currently.

When Unity mono + xbuild is autodetected.
User project is opened just fine but all C# 7+ features turn red.

No warns. So it might be confusing.
project has `<LangVersion>latest</LangVersion>`.