Implement replacement bbcode parser for authoring lesson content

[Razoric480]

Fixes #1188. Supercedes #1191.

Making some edits to the existing lessons was pretty slow. The point of this is to have an upgrade path to a new format that will replace the Godot resources, get rid of the editor plugin for authoring, and improve iteration speed when working on the content.

We can still have editor tools later on but now it would become a simpler list of lessons and code editor with syntax highlighting for BBCode tags instead of a more complex interface with blocks we drag and drop.

Changes

• Deletes course.tres in favour of a GDScript file, CourseLearnGDScriptIndex, that acts as an index of paths, course ID, and course title.
• Deletes all of the lesson.tres files in favour of a lesson.bbcode file from which lessons are built from
  • Amends export_presets to export BBCode lesson files
  • Currently converts the BBCode files into an intermediary lesson resource at runtime, but this will be changed by Remove old resource scripts and build all pages with BBCode parsing #1244 to build the pages and practices entirely from BBCode.
• Deletes the course-builder addon, since we're no longer generating resources outside of runtime and courses are built by BBCode