`wil::reg::try_get_value()` is gated by `_OPTIONAL_` and `__cpp_lib_optional`, but can't be re-included

WIL has several features where the features available from <wil/feature.hpp> depend on which headers have already been included; for the majority of these, you can include the header again, but this doesn't work for <wil/registry.h> as the entire file is gated with a traditional include guard for __WIL_REGISTRY_INCLUDED

Concretely, given how other parts of WIL work, I would expect this to work but it does not:

#include <wil/registry.h> // << works if I delete this line
#include <optional>
#include <wil/registry.h>

// ...

auto x = wil::reg::try_get_value<T>(key, name);

This is problematic when other a different header includes <registry.h>.

Suggested fixes:

• use the same pattern you're using in other parts of WIL to allow re-inclusion
• alternatively/additionally, add support for 'tweak headers'

As an example of tweak headers, wil/registry.h could start with:

#if __has_include(<wil-reg.tweaks.h>)
#include <wil-reg.tweaks.h>
#endif

I could then make a wil-reg.tweaks.h be included in my build, and include <optional> in that header.

Start with wil/registry.h and compare its include-guard and feature-gating pattern with the re-inclusion behavior described for wil/feature.hpp. Reproduce the shown include order with and verify that wil::reg::try_get_value(key, name) is available. Done means the header supports the intended include order or an explicitly documented tweak-header configuration.