We have found that a very common pattern for event handlers is
to capture a weak reference into a lambda, and in the event handler,
try to upgrade the weak reference to a strong one, and if so, do some work:

```cpp
widget.Closed([weak = get_weak(), data](auto&& sender, auto&& args)
{
    if (auto strongThis = weak.get())
    {
        strongThis->do_all_the_things(data);
    }
});
```

This commit extends the existing delegate constructors to permit a
`winrt::weak_ref` + lambda (or `std::weak_ptr` + lambda), which
simplifies the above to

```cpp
widget.Closed({ get_weak(), [this, data](auto&& sender, auto&& args)
{
    do_all_the_things(data);
} });
```

## Implementation notes

A lambda and pointer to member function are hard to distinguish
in a template parameter list. In theory, we could use SFINAE or
partial specialization, but a simpler solution is to distinguish
the two inside the body of the constructor, via
`std::is_member_function_pointer_v`.

The `com_ptr` and `shared_ptr` variants of the test were
unified, since I found myself editing two nearly identical tests.

Fixes microsoft#1371