Show warnings if code isn't indented properly

It would be great if students were advised on proper indentation in an automated fashion. It would help them get in the habit of formatting code cleanly much sooner, and help them find obvious bugs in their own code.

From our conversation over Slack, it sounds like we could infer what indentation type the code is primarily using, and enforce that everywhere in the code.

Maybe we wouldn't want to block the page from loading when indentation errors are present, but instead add a warning annotation to the line that's improperly indented.

该 issue 未指定任何文件或测试；首先定位编辑器中负责加载代码和进行行注释的入口点。确定如何推断缩进样式，以及如何在不阻塞页面的情况下显示警告，然后验证缩进不正确的行会被注释，而有效代码不受影响。