StringToClassRule: Add option to require strict casing

[DieterHolvoet]

My use case: I have some instances of the very simple 'wkt' string and an old school, non-namespaced library declaring a \WKT class. This results in multiple false positives.

I disabled this option by default for backwards compatibility. Another option would be to not make it configurable and require strict casing for all classes, not only internal classes.

[Slamdunk]

Hi, can you add tests for this enhancement?

I've just fixed the build, so also please rebase DieterHolvoet:feature/string-to-class-strict-casing