Implement scope control for the DSL

[nesk]

https://kotlinlang.org/docs/type-safe-builders.html#scope-control-dslmarker

becomes necessary when writing this kind of code:

labels { // this: Validatable<List<String>>
    hasSizeLowerThanOrEqualTo(3) // OK

    each {
        hasLengthGreaterThanOrEqualTo(1) // OK
        hasSizeGreaterThanOrEqualTo(1) // OK BUT SHOULDN'T BE (applied to the list, not to the string)
    }
}