extensionUtils: Move ExtensionState definition here

extensionUtils: Move ExtensionState definition here

It makes sense to keep extension-related enums in the same module instead
of spreading them between ExtensionSystem and ExtensionUtils.

More importantly, this will make the type available to the extensions-prefs
tool (which runs in a different process and therefore only has access to
a limited set of modules).

https://bugzilla.gnome.org/show_bug.cgi?id=789852