Extra: do not allow ambiguous conditions

This new PHPCS 3.9.0 sniff will flag code which contains a `||` and a `&&` operator in the same condition and their precedence is not clarified via parenthesis.

This should allow for users of WPCS to find/prevent some bugs in code.

I'd personally would **_highly_** recommend for WP Core to also use this sniff, but I suspect that will need due discussion/Make post/handbook change.

Note: running this sniff on WP Core as-is, would (to my surprise) not actually trigger any new errors.

WordPress-Extra/ruleset.xml

@@ -41,6 +41,10 @@
 		 https://github.com/WordPress/WordPress-Coding-Standards/pull/809 -->
 	<rule ref="Squiz.PHP.DisallowSizeFunctionsInLoops"/>
 
+	<!-- Do not allow ambiguous conditions.
+		 https://github.com/WordPress/WordPress-Coding-Standards/issues/2429 -->
+	<rule ref="Generic.CodeAnalysis.RequireExplicitBooleanOperatorPrecedence"/>
+
 	<!-- Check that functions use all parameters passed.
 		 https://github.com/WordPress/WordPress-Coding-Standards/issues/1510 -->
 	<rule ref="Generic.CodeAnalysis.UnusedFunctionParameter">