Function validateLessonRules

The semantic rules and author lints of a lesson, for input that already has the schema's shape (checked by validateLesson or by the consumer's own shape check). Returns { valid, errors, warnings } exactly as validateLesson does for a structurally valid lesson; does not throw on such input. options.extensions registers ext: exercise types and options.sourceLanguage names the set's source language, as for validateLesson.