libexpr: warn when encountering IFD with warn-import-from-derivation

This is intended to be a softer version of
`allow-import-from-derivation`, allowing for users to only flag when IFD
is encountered rather than error out

Based-on: https://github.com/NixOS/nix/pull/13279
Co-authored-by: gustavderdrache <alex.ford@determinate.systems>
Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
Change-Id: I879a444e3a24b4e82a8cea3a5a94834c78c3924a

doc/manual/change-authors.yml

@@ -99,6 +99,9 @@ goldstein:
   forgejo: goldstein
   github: GoldsteinE
 
+gustavderdrache:
+  github: gustavderdrache
+
 horrors:
   display_name: eldritch horrors
   forgejo: pennae

doc/manual/rl-next/warn-import-from-derivation.md

@@ -0,0 +1,12 @@
+---
+synopsis: "Emit warnings when encountering IFD with `warn-import-from-derivation`"
+prs: [nix#13279]
+cls: [3879]
+category: Features
+credits: [getchoo, gustavderdrache, edolstra]
+---
+
+Instead of only being able to toggle the use of [Import from
+Derivation](https://nix.dev/manual/nix/stable/language/import-from-derivation) with
+`allow-import-from-derivation`, Lix is now able to warn users whenever IFD is encountered with
+`warn-import-from-derivation`.