Create a component for lesson links

[NathanLovato]

Code a component to link to lessons in the course and validate the links: <LessonLink course="learn_2d_gamedev_godot_4" href="module/lesson_slug" label="the lesson" />

The point is to:

1. Validate that links are correct (the lesson exists) and catch when they change
2. Automatically insert the lesson title if no label is provided

So this should both do link validation and help with not looking up courses.

To decide: should this use file paths or actual URLs from the final website?