ProperEscapingFunction: make the $escaping_functions property protected

The `public` `$escaping_functions` property was never _intended_ for allowing it to be changed via a custom ruleset.

As the array _format_ of the property has now changed, adjusted values for this property set in a custom ruleset will no longer work.

This is a breaking change, but as:
1. The property was never intended to be changed from a custom ruleset and
2. The chances of any custom rulesets actually existing which _do_ change this property being very, very small...

I believe it is justified to make the breaking change in the format in a minor release as the format change provides a real functional benefit.

Now, as this breaking change will be made anyway, we may as well do it right and mark the property as `protected` preventing it from ever being changed via a custom ruleset.

Author: jrfnl

WordPressVIPMinimum/Sniffs/Security/ProperEscapingFunctionSniff.php

@@ -23,7 +23,7 @@ class ProperEscapingFunctionSniff extends Sniff {
 	 *
 	 * @var array
 	 */
-	public $escaping_functions = [
+	protected $escaping_functions = [
 		'esc_url'    => 'url',
 		'esc_attr'   => 'attr',
 		'esc_attr__' => 'attr',