Replace content in conteneditable

[nucleartux]

Currently, the behavior in "textarea" and "contenteditable" elements is different.
When we click the extension button in a "textarea," it replaces the content.
However, when we click the same button in a "contenteditable" element, it copies fixed content to the clipboard.

This is done because I don't know how to replace content in a "contenteditable" element, or if it's even possible.