Support for UnmanagedCallersOnlyAttribute Class.

**Is your feature request related to a problem? Please describe.**

The Vulkan API provides a debug listener which requires a user-provided native callback method to handle the debug messages.

**Describe the solution you'd like**

I would like support for the UnmanagedCallersOnlyAttribute Class to be added to the FSharp Compiler so that Vulkan users can use UnmanagedCallersOnly to implement the callback method in FSharp like they can in CSharp.

**Describe alternatives you've considered**

The workaround is to add a CSharp project to our FSharp software just to get this debugging feature.

Thank you in advance,
Dean.