feat: port Algebra.Star.Subalgebra (#2819)

Note: Lean was continually crashing on me as I was porting this file, I believe sure to autoparams.

- [x] depends on: #2817

Author: j-loreaux

Mathlib.lean

@@ -237,6 +237,7 @@ import Mathlib.Algebra.Star.Pointwise
 import Mathlib.Algebra.Star.Prod
 import Mathlib.Algebra.Star.SelfAdjoint
 import Mathlib.Algebra.Star.StarAlgHom
+import Mathlib.Algebra.Star.Subalgebra
 import Mathlib.Algebra.Star.Unitary
 import Mathlib.Algebra.Support
 import Mathlib.Algebra.Tropical.Basic