Add `static let allImages` array to namespaced image folders

[apptekstudios]

This addition allows us to safely get an array of all the images contained within a namespaced folder

• it was chosen to only make this available for namespaced folders, as this indicates these images are semantically grouped
• this is particularly useful where we want to dynamically (or randomly) pick an image from this group whilst maintaining the safety of R.swift