I got this question in a comment recently:
I have been asked this question a few times, so this might be a good opportunity to clarify.
The inline editor uses Firebase cloud functions
The code editor inside the Dialogflow console, also called as the Inline Editor, is actually used only for writing Firebase cloud functions. And even the type of code you can write within the inline editor using cloud functions is a bit limited. (For example, right now, if you want to write code that saves to a database you cannot do so from the inline editor, to the best of my understanding. You need to go to the regular webhook in that case.)
The inline editor in Dialogflow is intended to be a very simple, quickstart tool and can only be used for writing NodeJS code at the time of this writing.
In all likelihood, you will graduate to using the regular webhook fairly quickly once you get an understanding of how webhooks work.