Make a function to have a better syntax for snapshotting variable files in a channel emitting a directory

[maxulysse]

I managed to make a nice and easy function that works, but not recursively.
I have something that works recursively and I do think it looks slightly better, but let's be honest it really is just meh.
Having a function with nft-utils would render this usecase much simpler and nice looking (and fully extendable to any use case (ie with or without recursion))
cf:

• Update snapshotting-variable-files-in-a-channel-emitting-a-directory website#3055
• simplify assertion for stable files in a folder modules#7462