Short answer:
Click whichever script(s) you care about and select 'Create Override' in the context menu:
Then create a new override in the
core resource collection:
Then you'll be able to edit it as you expect.
Longer answer:
It's hard for us to convey this in the UI, but the new configuration model in Ignition 8.3 is basically a tree structure.
It's essentially a single project inheritance tree, where the order is hardcoded.
What you do have control over is which deployment mode (if any) is loaded, and where you define resources.
You cannot, by definition, edit resources in system - they are provided by either the platform itself or a module. This includes the base call scripts. So what you have to do is "override" them, creating a duplicate in the core collection (or whichever deployment mode you have active, if you have one) that is then a "regular" resource you own and can modify as you wish.