OneObjectStructurePerFile: move from Extra to Core

Per the discussion had on Slack about this initiated by GaryJones.
WP Core currently has 27 violations against this rule.

In `Extra` the error message was changed and downgraded to a `warning`.

Now the sniff will be added to `Core`, per this PR, the:
* Message text will still be changed a little to be clearer, but will be stricter.
* The message will be an `error` and not be downgraded to a `warning`.

To do:
- [ ] Add a rule to this effect to the handbook.

WordPress-Core/ruleset.xml

@@ -517,6 +517,12 @@
 		<severity>0</severity>
 	</rule>
 
+	<!-- Encourage having only one class/interface/trait per file.
+		 Moved from Extra to Core after discussion on Slack. -->
+	<rule ref="Generic.Files.OneObjectStructurePerFile">
+		<message>Best practices: Declare only one class/interface/trait in a file.</message>
+	</rule>
+
 	<!--
 	#############################################################################
 	Not in the coding standard handbook: WP specific sniffs.

WordPress-Extra/ruleset.xml

@@ -56,12 +56,6 @@
 	<!-- https://github.com/WordPress/WordPress-Coding-Standards/pull/382#discussion_r29981655 -->
 	<!--<rule ref="Generic.CodeAnalysis.UnusedFunctionParameter"/>-->
 
-	<!-- Encourage having only one class/interface/trait per file. -->
-	<rule ref="Generic.Files.OneObjectStructurePerFile">
-		<type>warning</type>
-		<message>Best practice suggestion: Declare only one class/interface/trait in a file.</message>
-	</rule>
-
 	<!-- Verify modifier keywords for declared methods and properties in classes.
 		 https://github.com/WordPress/WordPress-Coding-Standards/issues/1101 -->
 	<rule ref="Squiz.Scope.MethodScope"/>