Sitecore Powershell Extensions provides a lot of integration points with Sitecore pipelines, ribbons and ect. Read more about all integration poins supported by SPE.
Last time I working on SPE integration with Experience editor and I notice that it doesn’t work with Sitecore 9.0.1 – you can read more in post Sitecore maintenance message hard to miss.
I did a quick verification and it seems that integration pipeline is in a wrong place. Below you can find a proper configuration PageEditorNotificationScript is inside <getPageEditorNotifications>.
I checked and notice that <getPageEditorNotifications> in Sitecore 9 is inside a <group> tag.
I created a pull request in SPE repository.