Added a convenience is function for optional CasePathable values

[HarshilShah]

This PR is aimed at improving ergonomics for is checks specifically in TCA. Destinations and other pieces of CasePathable state can often be optional in TCA, and so out of the box the CasePathable.is function requires either unwrapping the optional state or tacking on == true to compile. I defined this extension in my own codebase a while ago to make that syntax a bit nicer to read, so as I was copying it over to another project I thought it might be worth submitting for inclusion right in the library itself

[stephencelis]

Hey @HarshilShah! Thanks for opening the PR as it made a deficiency clear to me 😄

Technically this API isn't actually needed, because Optional is CasePathable, and because you can squash \.some chaining when forming case key paths. You can confirm this by compiling the following today:

@CasePathable enum Foo { case bar }

func f() {
  let optional = Optional(Foo.bar)
  optional.is(\.bar)
}

I think the problem is that Xcode autocomplete will favor optional-chaining into the Wrapped over applying the method directly, so you get the following by default:

optional?.is(\.bar)

So I think this overload is worth adding to the library if only to make the autocomplete experience better 😄

Thanks again!