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.

该 issue 未指明任何文件、测试或入口点。首先定位现有的积木工具箱及其配置，然后将其与提议的初学者和高级积木集进行比较；当渐进式语言级别能够选择性地替换或扩展工具箱时，这项工作就完成了。

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