Language levels for blocks

A problem with my current block-based class is that there's just too much in the toolbox, and students can't find what's important. I'd like to instead set up progressive language levels. This would be an option that lets you swap out the toolbox for different sets of blocks. For the beginning of the class, we might just have a few blocks: `drawingOf`, `&`, `circle`, `solidCircle`, `rectangle`, `solidRectangle`, `translated`, `rotated`, `scaled`, and `colored` -- and a second drawer of major colors. Then later, we could selectively enable features like variables, polygons, lists and list comprehensions, and animations.

Prior art includes Racket, and others.

The issue names no files, tests, or entry points. Start by locating the existing block toolbox and its configuration, then compare it with the proposed beginner and advanced block sets; the work is complete when progressive language levels can selectively replace or extend the toolbox.

Written by the indexing model from the issue text.