Core: Add deprecation warning to 'date' method

Ref #1787

src/core.js

@@ -1386,6 +1386,16 @@ $.extend( $.validator, {
 
 		// https://jqueryvalidation.org/date-method/
 		date: function( value, element ) {
+			if ( this.settings.debug && window.console ) {
+				console.warn(
+					"The `date` method is deprecated and will be removed in version '2.0.0'.\n" +
+					"Please don't use it, since it relies on the Date constructor, which\n" +
+					"behaves very differently across browsers and locales. Use `dateISO`\n" +
+					"instead or one of the locale specific methods in `localizations/`\n" +
+					"and `additional-methods.js`."
+				);
+			}
+
 			return this.optional( element ) || !/Invalid|NaN/.test( new Date( value ).toString() );
 		},