Blocks should have descriptive icons

It would be a huge improvement if blocks had descriptive icons that illustrated what they did. This is especially true when the tool is used to teach younger students for whom reading words like "rotate" is not quite second nature.

I could work on making these as images; but I've also just switched over the UI icons on the main site to https://materialdesignicons.com/ which has a lot of icons in SVG format that might be useful?

首先检查主站现有的 UI 图标集成和块渲染入口；issue 中没有指定源文件或测试。比较适用于这些块的 Material Design Icons SVG，然后确认每个块都有一个易于识别的描述性图标，并且教学 UI 仍然可用。

由索引模型根据 Issue 内容生成。