Count lessons, practices, quizzes, etc. at build time

[NathanLovato]

Once we have our MDX parser, for each course that we build, we should use the AST to count all the metadata we would like to have about individual courses like how many lessons and modules are out, how many practices are there, and quizzes.

This should be output as a JSON file in the course directory.

This data could then be used in early access roadmaps to always give students and visitors up to date information about the courses: https://school.gdquest.com/products/godot-4-early-access#roadmap

NOTE: This count should be based on released/published lessons. So, it probably depends in part on being able to parse index.ts files and creating a course index in this build system