Broke out `CSSelectorValidator`.

This is the third of an expected series of five VERY incremental changes breaking out the content-selection validation classes from the `ContentSelection` class proper.
Broke out `CSNodeIndexValidator`.

This is the fourth of an expected series of five VERY incremental changes breaking out the content-selection validation classes from the `ContentSelection` class proper.
Broke out `CSTextOffsetValidator`.

This is the Fifth of an expected series of five VERY incremental changes breaking out the content-selection validation classes from the `ContentSelection` class proper. We’ll get back to fleshing out the `ContentSelection` `setEnd` and `getContent` methods momentarily.