Implement IGoToBaseService

This is the interface:

```csharp
using System.Threading.Tasks;
using Microsoft.CodeAnalysis.FindUsages;
using Microsoft.CodeAnalysis.Host;

namespace Microsoft.CodeAnalysis.Editor.GoToBase
{
internal interface IGoToBaseService : ILanguageService
{
///
/// Finds the base members overridden or implemented by the symbol at the specific position in the document,
/// pushing the results into the context instance.
///
Task FindBasesAsync(Document document, int position, IFindUsagesContext context);
}
}
```

* Implement an F# version of the interface exposed via the shim