Add type_caster<std::monostate>

[laramiel]

Description

std::monostate is a tag type that allows std::variant to act as an optional, or allows default construction of a std::variant holding a non-default constructible type.

Here a type_caster<std::monostate> is added that specializes void_caster<>, as well as tests for the same.

Suggested changelog entry:

``type_caster<std::monostate>`` was added. ``std::monostate`` is a tag type that allows ``std::variant`` to act as an optional, or allows default construction of a ``std::variant`` holding a non-default constructible type.

[rwgk]

Oops, LGTM but the CI isn't happy. I'll look again when it is green.

[laramiel]

Sorry about that; did most of these edits directly in the browser.

[rwgk]

Thanks Aaron!