feat(c#): Multi-return support

[jsturtevant]

This adds the codegen test for multi-return with c#. It creates an interface like:

namespace wit_the_world.wit.exports.foo.foo.MultiReturn;

public interface IMultiReturn {
    static abstract void Mra();
    
    static abstract void Mrb();
    
    static abstract uint Mrc();
    
    static abstract uint Mrd();
    
    static abstract Tuple<uint, float> Mre();
    
}

and export interop looks like:

[UnmanagedCallersOnly(EntryPoint = "foo:foo/multi-return#mre")]
    public static int wasmExportMre() {
        
        Tuple<uint, float> result =  MultiReturnImpl.Mre();
        uint result0 = result.Item1;
        float result1 = result.Item2;
        
        var ptr = returnArea.AddrOfBuffer();
        returnArea.SetS32(0, unchecked((int)(result0)));
        returnArea.SetF32(4, result1);
        return (int)(ptr);
        
    }

[yowl]

There's a bit of overlap here with #808 . I went with the (type1, type2, ..) syntax rather than Tuple, thinking that at some point we could name the elements rather than just Item1, Item2 etc.

[jsturtevant]

@yowl I've rebased on top of your changes for #808 and we are good to go. Thanks @silesmo for the float fix too!

[yowl]

Looks good, thanks