[Merged by Bors] - feat: port ModelTheory.Substructures

Mathlib.lean

@@ -1592,6 +1592,7 @@ import Mathlib.ModelTheory.Basic
 import Mathlib.ModelTheory.Encoding
 import Mathlib.ModelTheory.LanguageMap
 import Mathlib.ModelTheory.Semantics
+import Mathlib.ModelTheory.Substructures
 import Mathlib.ModelTheory.Syntax
 import Mathlib.NumberTheory.ADEInequality
 import Mathlib.NumberTheory.Basic