Add CSS.escape() to DOM searching chapter

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)

Start by locating the “Searching: getElement, querySelector” chapter mentioned in the issue and review its existing querySelector examples. Check the linked MDN CSS.escape() reference for accurate guidance; done means the chapter clearly explains passing a variable safely to querySelector and includes a suitable example.

Written by the indexing model from the issue text.