Improve extendability of validator rules

[wimski]

Resolves #851

Yes. Accepting PRs, but preferrably one that opens up all validation rules for extensibility to ensure consistency.

[mfn]

I love opening up the things, aligns with my philosophy.

I guess however it's important to keep in mind that only @api marked elements are covered by semver/backwards compatibility promises.
(I kind of made this up from observation on this repository, could not quickly find a proper doc if this is the case -> all my search for @api only yielded code-use)

[spawnia]

There is more work to be done with those classes in regards to strict typing. At least the properties are done now and the member visibility is fine, so this is a good point to merge as is.