Core: Implement .even() & .odd() to replace POS :even & :odd

`:even` & `:odd` are deprecated since jQuery 3.4.0 & will be removed in 4.0.0.
The new `even()` & `odd()` methods will make the migration easier.

Closes gh-4485

(cherry picked from commit 78420d427c)

src/core.js

@@ -105,6 +105,18 @@ jQuery.fn = jQuery.prototype = {
 		return this.eq( -1 );
 	},
 
+	even: function() {
+		return this.pushStack( jQuery.grep( this, function( _elem, i ) {
+			return ( i + 1 ) % 2;
+		} ) );
+	},
+
+	odd: function() {
+		return this.pushStack( jQuery.grep( this, function( _elem, i ) {
+			return i % 2;
+		} ) );
+	},
+
 	eq: function( i ) {
 		var len = this.length,
 			j = +i + ( i < 0 ? len : 0 );