Improve JavaScript code editor in item preprocessing

The idea came from ZBXNEXT-4944. 

When you open JavaScript code editor it is not clear where exactly you are adding the code. Is it function body or elsewhere?

It would be nice to have code blocks in editor that are wrapping user input into UI. It helps to better understand the idea without even opening the documentation. Additionally, there could be minimal working code example "return value;" as placeholder. And since we actually use anonymous function it could be something like this (thanks for the screenshot, vjaceslavs):