Add support for providing a numeric difficulty class parameter on action use

[nikolaj-a]

Providing just a number for a check DC is such a common use-case that it warrants direct support, instead of having to wrap it up in an object. The original syntax, as well as a DCSlug, is still a valid value.

Before

game.pf2e.actions.get("climb").use({ difficultyClass: { value: 31 } });

After

game.pf2e.actions.get("climb").use({ difficultyClass: 31 });