AK: Let HashMap export its key and value types

Author: https://github.com/alimpfard
Commit: https://github.com/SerenityOS/serenity/commit/5bf37d758cb
Pull-request: https://github.com/SerenityOS/serenity/pull/7255
Reviewed-by: https://github.com/linusg

AK/HashMap.h

@@ -33,6 +33,9 @@ private:
     };
 
 public:
+    using KeyType = K;
+    using ValueType = V;
+
     HashMap() = default;
 
 #ifndef SERENITY_LIBC_BUILD