Generate C# extension members for type extensions

Hi,

A library written in F# does not expose type extensions done in other modules - see discussion here https://github.com/dotnet/fsharp/discussions/20032 and my example code here https://github.com/ernestasjuska/FSharpIssues.TypeExtensionsDemo/blob/main/TypeExtensionsDemo/Program.cs.

Some of the problem is that type extensions generate method names like `Thing.get_Texture` that are not visible from C# code.
For now, the workaround is to write missing extension members on C# side by hand (or source generate).

https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/type-extensions only describes the old extension methods way using `[]` attribute.

So, I wish there was something like `[]` attribute that could be applied to F# type extension so type extension would be available in C# code.

This is low priority and not critical.

Thanks.

P.S. I already tried to see if it is possible to at least write extension members code manually. From C# end it needs reflection so that is not nice (but it is very easy) and from F# I hit the limitation that types cannot be nested so I could not mimic the extension members markers that are generated inside assembly DLL.

Start with the linked discussion and the TypeExtensionsDemo/Program.cs example to understand the C# visibility problem, then compare it with the linked F# type-extensions documentation. Define the desired ExtensionMembers behavior and verify that F# type extension members can be consumed from C# without hand-written wrappers.

Written by the indexing model from the issue text.