Windows CI using DkML

[jonahbeckford]

@tahina-pro

This is a PR to enable fstar to build on Windows/OCaml CI. It uses DkML, although I wrote it in a way that should be able to run with opam 2.2 (I think!).

Tasks open:

• [x] Currently there is a DISABLE_FSHARP=1 variable for make package so that it skips the dotnet build (I don't have dotnet on my machine).

~~I am submitting the PR now even with the open task to make sure the self-hosted CI runner used by fstar works. I can add back in dotnet in a separate commit (but still in this PR).~~

DONE. ~~So don't merge this PR until you see the task list closed!~~