Implemented is OpaqueType method

[Pask423]

Solves opaque types task in #296

[adamw]

We definitely need a test :)

[KacperFKorban]

The problem with opaque types is actually a bit more complex. Magnolia for Scala 3 is pretty much just a wrapper for Mirrors (at least for now) and Mirrors aren't generated for opaque types. And I'm pretty sure that it was a design decision. This means that magnolia cannot derive instances for opaque types.
These changes seem to work as intended, but at the current state they will always result in isOpaque = false.
They will be very useful once #321 works, but unfortunately, I haven't had time to work on it lately.
Not sure if we should hold this PR or merge it anyway with the intent for it to work in the future. @adamw?

[adamw]

I think it's better to hold off with this PR, as it would give the false impression that this feature works, while it doesn't

We should probably concentrate on macrolia then :)