Add an invariant check for Titlecase_Mapping

[eggrobin]

Something that would have caught this: https://github.com/unicode-org/unicodetools/pull/554#issuecomment-1745562305.

One possibility is that the Titlecase_Mapping is not to itself except for Lt and Georgian Ll; maybe we can be more specific?

[markusicu]

If it's not gc=Lt and the decomposition mapping is to a sequence of cased letters, then it should have a Titlecase_Mapping?

[markusicu]

For most characters, uppercase = titlecase. That alone should have helped with #554.