Add CSS.escape() to DOM searching chapter

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Hello, I was wondering if it would be worth adding something in the chapter _Searching: getElement, querySelector_ about how to pass a variable through `document.querySelector` by using `CSS.escape()`, ie `document.querySelector('div.' + CSS.escape(userColor))`

I write this because I just spent a while trying template literals, etc, and I had no idea CSS.escape() existed and is necessary to do this.

It's probably pretty uncommon to need to pass variables to querySelector, but if anyone else thinks it's worth adding I would be happy to make a PR.

[https://developer.mozilla.org/en-US/docs/Web/API/CSS/escape](url)

首先定位 issue 中提到的“Searching: getElement, querySelector”章节，并查看其中现有的 querySelector 示例。检查链接的 MDN CSS.escape() 参考文档以获取准确的指导；完成的标准是该章节清楚地解释如何安全地将变量传递给 querySelector，并包含一个合适的示例。

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