Fix broken textarea auto size

[Smankusors]

The code is taken from https://stackoverflow.com/questions/454202/creating-a-textarea-with-auto-resize

Before and after on operators.html:
Before:

After:

You can try this out online at https://json-logic.smankusors.com

---

Btw, there's an alternative way without using JS, which is to use only CSS:

textarea {
   field-sizing: content;
}

However, as of now, it's only available on Chrome 123 and newer, so I think it's too new for us to use.