Added a check for use of type variable to TypeToken constructor.

[JornVernee]

A pull request that addresses my earlier issue #1219.

This adds a check to the protected TypeToken constructor that will now throw a runtime exception if the TypeToken uses a type variable. So that this design time error produces a more clear error earlier during execution.

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[googlebot]

CLAs look good, thanks!

[JornVernee]

I've used a RuntimeException because getSuperclassTypeParameter does the same for missing type parameters. So I think that issue can be ignored?

I've also made the accessibility of the usesTypeVariable method the same as getSuperclassTypeParameter. That could also be made private not sure what to do there.

I also haven't added any javadoc yet. I see you are not using javadoc annotations, so I'm not entirely sure what to do there either. I also haven't added myself as an author in the javadoc yet, not sure if that's ok? Or if it starts at a certain level of contribution.

[JornVernee]

Closing old PR