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 78420d4)

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 );

test/unit/core.js

@@ -670,6 +670,18 @@ QUnit.test( "first()/last()", function( assert ) {
 	assert.deepEqual( $none.last().get(), [], "last() none" );
 } );
 
+QUnit.test( "even()/odd()", function( assert ) {
+	assert.expect( 4 );
+
+	var $links = jQuery( "#ap a" ), $none = jQuery( "asdf" );
+
+	assert.deepEqual( $links.even().get(), q( "google", "anchor1" ), "even()" );
+	assert.deepEqual( $links.odd().get(), q( "groups", "mark" ), "odd()" );
+
+	assert.deepEqual( $none.even().get(), [], "even() none" );
+	assert.deepEqual( $none.odd().get(), [], "odd() none" );
+} );
+
 QUnit.test( "map()", function( assert ) {
 	assert.expect( 2 );