feat: introduce UnwrapOpaque type

[jmike]

This PR introduces a new UnwrapOpaque type to remove the [tag] from Opaque values.

Why is this necessary?

1. Use an opaque type as object keys
2. Prevent stupid TS4058 error, i.e. Return type of exported function has or is using name X from external module Y but cannot be named.

Fixes #165

[sindresorhus]

Any thoughts on the discussion in #165?

[jmike]

As far as naming goes, I do not have a preference. I can amend this PR to anything you decide.

Re: protected, I don't like classes but they seem to do the trick here. I cannot say - needs extensive testing.

I just thought this discussion is ongoing for quite some time, so why not do something about it since I am also affected by the same problems at work.

[jmike]

Hey, I don't want to push here or anything... if you don't want to merge this it's fine. Just kindly let me know because we need this functionality, so I might as well find/create another lib for branded types only.

[sindresorhus]

I'm fine with adding this, but it should be called UnwrapOpaque.

[jmike]

I have renamed the new type (as well as the PR title and description) to UnwrapOpaque per @sindresorhus suggestion.

[sindresorhus]

Looks good.

You need to add it to the readme.

[jmike]

Cool, the code has been updated accordingly.