Define reserved types example, invalid, and test

[immqu]

VERS currently lacks types reserved for special use, similar to RFC 2606, Section 2, “TLDs for Testing”.

Motivation:

• A reserved example type is needed for documentation, so examples can be used without risking a collision with a real or future type registration
• A reserved invalid type would allow documentation and tests to represent values that are intentionally invalid
• A reserved test type is needed for test cases. Currently, a test might use some made-up "unknown-type", but this is not stable: the name could be registered in the future, changing the expected behavior of the test

Having these types reserved means we could use these types in a syntactically correct way for documentation and testing.