Add validation constraints to file.settings config

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rebased and left question regarding possible solition for file vs directory

All is green, gonna set to needs review for visibility on the one part that still needs decision based on review.

So the question on the MR is if we can split out a config schema for a file vs a directory. I think that is always different. I don't know of any places in core (or contrib) where you have to enter a path to a directory-or-file. So splitting these into two different schemas makes sense to me.

Not 100% positive on what this means;

Add new types?

type: directory
type: file_uri

And/or constraints to verify existence?

File?
Directory?

Add new types?

type: directory
type: file_uri

Yes, those 2 new types should be made, and those types should get a constraint.
For file_uri it could be a file_exists check (or that symfony constraint I guess). For directory it should either exist or be in a writable place?

Should this be moved back to NW?