chore: fix implicit-reducible diamond in Lie subalgebras (#38260)

The following example fails on master, is fine on the branch:
```
example (I : LieIdeal R L) : ((LieIdeal.lieRing R L I).toBracket : Bracket ↥I ↥I) =
    (LieIdeal.lieRingModule (↥I) I).toBracket := by
  with_reducible_and_instances rfl
```

Co-authored-by: sgouezel <sebastien.gouezel@univ-rennes1.fr>

Author: sgouezel

Mathlib/Algebra/Lie/Ideal.lean

@@ -72,6 +72,12 @@ theorem LieIdeal.toLieSubalgebra_toSubmodule (I : LieIdeal R L) :
     ((I : LieSubalgebra R L) : Submodule R L) = LieSubmodule.toSubmodule I :=
   rfl
 
+instance LieIdeal.bracket {R L : Type*} [CommRing R] [LieRing L] [LieAlgebra R L]
+    (I : LieIdeal R L) [Bracket L M] : Bracket I M where
+  bracket x m := ⁅(x : L), m⁆
+
+instance (I : LieIdeal R L) : Bracket I I := inferInstance
+
 /-- An ideal of `L` is a Lie subalgebra of `L`, so it is a Lie ring. -/
 instance LieIdeal.lieRing (I : LieIdeal R L) : LieRing I :=
   inferInstanceAs <| LieRing I.toLieSubalgebra