md2html exercise generator

[ ] Add txtar file format
[ ] Initial Fundamentals course content
[ ] Format all evy files, including inside txtar files
[ ] Deploy to learn.evy.dev static content only
[ ] Prettier output from learn/content -> frontend/learn
[ ] Workout unittests and quizzes (question href/image rewriting)
[ ] Quiz, unittest question selection update: Round-robin from the different exercises to ensure all exercises are covered.
[ ] Don't validate question composition if there are sealed / ignoreSelaed questions.
[ ] Build a server component for single-user
[ ] Add JS for question selection and randomization rewriting)
[ ] Build a server component for single-user
[ ] Add JS for question selecton and randomization

[ ] Remove xmlns="http://www.w3.org/2000/svg" from inline svg
[ ] Add width="200px" height="200px" to renderer.go for svg file output, remove for inline.
[ ] Use ANSWER as part of question path, e.g. dot/{ANSWER}.evy

[x] evy · Question => model.Name
[x] Remove <hr>
[x] Add table for composition to Exercise.
[x] Create unit Model; output model with unit badge box
[x] Create quiz Model
[x] Create unittest Model
[x] Create course Model; output model with unit badge boxes
[x] Track exercise and unit names (derive from h1 or add as frontmatter field)
[x] Finish catalog.json
[x] Re-arrange validateModelPaths to come after newModels
[x] Update validateModelPaths for: 1 course, 1 unit, 1 unittest, 1 exercise per dir.
[x] Fix cls command for buffer output
[x] Export (course) catalog
[x] Check learn/README.md
[x] Fix rsc.io/markdown replace

evylang / todo