Add "send file to F# interactive" command to MonoDevelop

I tested this with the newly released Xamarin 5.0.

I did not assign a keybinding for the command, but it does show up in the menu.

There is also a define for MD 4.2.5 in this PR, because I originally tested it with Xamarin 4.2.5, before 5.0 was released, and that define was necessary to make the old version build.