title: The SDK 'Microsoft.NET.Sdk.Web' specified could not be found in VSCode OmniSharp extension
body: <p>I've installed dotnet core 3 sdk and OmniSharp 1.18 extension on VSCode
  but OmniSharp has an error:
  Microsoft.Build.Exceptions.InvalidProjectFileException: The SDK 'Microsoft.NET.Sdk.Web' specified could not be found.</p>