Extra: Replace Squiz.Scope.MemberVarScope with `PSR2.Classes.Proper…

…tyDeclaration`

PR 1103 added the `Squiz.Scope.MemberVarScope` sniff to the `WordPress-Extra` ruleset which checks that all class properties have their visibility declared.

The upstream `PSR2.Classes.PropertyDeclaration` sniff also does this, however it contains more extensive checks.
In addition to checking for the visibility of properties being declared, this sniff also checks:
* That property names are not prefixed with an underscore `_` to indicate visibility.
    _For methods, we already check this in the `Extra` ruleset via the `PSR2.Methods.MethodDeclaration` sniff._
* That the `var` keyword is not used for property declarations.
    This could be considered a duplicate check with the "use visibility" check.
* That each property declaration statement only declares one property, i.e. it forbids the use of code like this:
    ```php
    public $prop1 = true,
        $prop2 = '',
        $prop3 = 123;
    ```
    As there is no precedent or reference to this type of property declarations in the handbook or elsewhere in the various rulesets, I'm not sure that we're ready to take a stand on this type of coding, so I've **excluded** this particular error code for now.
* As of PHPCS 3.4.0: that the order of the property keywords is `visibility static`, not `static visibility` (includes fixer).
    _For methods, we already check this in the `Extra` ruleset via the `PSR2.Methods.MethodDeclaration` sniff._

All in all, using the `PSR2` sniff is more in line with the checks we already have in place for methods, so IMO, this seemed like a good change to make.

WordPress-Extra/ruleset.xml

@@ -73,7 +73,9 @@
 	<!-- Verify modifier keywords for declared methods and properties in classes.
 		 https://github.com/WordPress-Coding-Standards/WordPress-Coding-Standards/issues/1101 -->
 	<rule ref="Squiz.Scope.MethodScope"/>
-	<rule ref="Squiz.Scope.MemberVarScope"/>
+	<rule ref="PSR2.Classes.PropertyDeclaration">
+		<exclude name="PSR2.Classes.PropertyDeclaration.Multiple"/>
+	</rule>
 	<rule ref="Squiz.WhiteSpace.ScopeKeywordSpacing"/>
 	<rule ref="PSR2.Methods.MethodDeclaration"/>
 	<rule ref="PSR2.Methods.MethodDeclaration.Underscore">