Add class to the current lesson in the Course Syllabus block

[brianhogg]

Is your feature request related to a problem? Please describe.
In older themes, using the Course Syllabus widget would add a current-lesson class on the current lesson. This lets the lesson be styled (ie. bolded) to show the student what lesson they are on. In the newer Course Syllabus block however this class (or a similar class) is not added.

Describe the solution you'd like
Add a CSS class on the div for the lesson.

Describe alternatives you've considered
There is a llms_get_preview_classes filter we could possibly use, which only passes in the $classes already generated currently. Or, creating a custom version of the lesson-preview.php template.

[brianhogg]

For some reason the global $post is the Course rather than the Lesson when the course syllabus is generating, so you can't do a if ( get_the_ID() === $this->get( 'id' ) ) within the LLMS_Lesson::get_preview_classes method.