feat: add Tagged for composable tagged types

[ethanresnick]

This adds two new types, Tagged and UnwrapTagged, per #665.

It also tweaks UnwrapOpaque so that it works on types produced by Tagged (as well as still working on types produced by Opaque, of course). This is a small gesture at interoperability between Opaque and Tagged, to make it easier for codebases to migrate from Opaque to Tagged.

Fixes #665

[sindresorhus]

This looks very good 👍

[sindresorhus]

You need to also add them to the readme.

[sindresorhus]

Some types like https://github.com/sindresorhus/type-fest/blob/main/source/exact.d.ts, https://github.com/sindresorhus/type-fest/blob/main/source/invariant-of.d.ts, and https://github.com/sindresorhus/type-fest/blob/main/source/conditional-pick-deep.d.ts have special handling of Opaque. That should probably extend to Tagged too.

[ethanresnick]

Ok, I think I addressed all the comments. For the three types you mentioned that use Opaque (Exact, InvariantOf, and ConditionalPickDeep), I did something different in each case:

• Exact: I extended the current approach to work with Tagged types too. As I mentioned in the inline code comment, I'm not sure the current approach is exactly correct/ideal, but that can be a topic for a separate PR.

• InvariantOf: I kept the same basic logic, but inlined it to remove the dependency on Opaque, in case we want to deprecate Opaque at some point. I also put the tag that's enforcing the invariance in a differently-named symbol, so that it won't conflict with tags added by Tagged/Opaque. (Before, it was impossible to do Opaque<InvariantOf<X>> or InvariantOf<Opaque<X>>, as both InvariantOf and Opaque would've tried to put conflicting types at the same phantom [tag] key.)

• ConditionalPickDeep didn't seem like it should be using opaque/tagged types at all; from my reading, it looks like it really just wanted a unique symbol type. So, I updated that accordingly, and all the tests still pass.

[sindresorhus]

Thanks ;)