Revert "Do not allow protocols to define structs nor exceptions, closes #14158"

Projects rely on this feature, therefore we have to revert
to avoid breaking changes.

This reverts commit 01c8202.

Closes #14803.

Author: josevalim

lib/elixir/lib/protocol.ex

@@ -770,15 +770,7 @@ defmodule Protocol do
 
         # We don't allow function definition inside protocols
         import Kernel,
-          except: [
-            def: 1,
-            def: 2,
-            defdelegate: 2,
-            defguard: 1,
-            defguardp: 1,
-            defstruct: 1,
-            defexception: 1
-          ]
+          except: [def: 1, def: 2, defdelegate: 2, defguard: 1, defguardp: 1]
 
         # Import the new `def` that is used by protocols
         import Protocol, only: [def: 1]