Emoji: Move printing of emoji loader script module from wp_head to wp_print_footer_scripts.

This removes ~3KB of HTML from the critical rendering path of markup in the `head`, thus marginally improving FCP/LCP in slower connections. It also fixes a Firefox issue with script modules by ensuring the emoji loader script module is printed after the `importmap`. 

Existing plugins that disable emoji by unhooking the action as follows will continue to work as expected:

    remove_action( 'wp_head', 'print_emoji_detection_script', 7 );

Additionally, some obsolete `DOMReady` and `readyCallback` logic was removed. A script module (as it has a deferred execution) only ever executes when the DOM is fully loaded. This means there was no need for a `DOMContentLoaded` event which was removed in [60899], and the remaining ready detection logic can be removed.

Follow-up to [60899].

Developed in #10145.

Props westonruter, wildworks.
Fixes #63842.
Fixes #64076.


git-svn-id: https://develop.svn.wordpress.org/trunk@60902 602fd350-edb4-49c9-b593-d223f7449a82

Author: westonruter

src/js/_enqueues/lib/emoji-loader.js

@@ -2,6 +2,8 @@
  * @output wp-includes/js/wp-emoji-loader.js
  */
 
+/* eslint-env es6 */
+
 // Note: This is loaded as a script module, so there is no need for an IIFE to prevent pollution of the global scope.
 
 /**
@@ -12,15 +14,13 @@
  * @property {?string} source.concatemoji
  * @property {?string} source.twemoji
  * @property {?string} source.wpemoji
- * @property {?boolean} DOMReady
- * @property {?Function} readyCallback
  */
 
 const settings = /** @type {WPEmojiSettings} */ (
 	JSON.parse( document.getElementById( 'wp-emoji-settings' ).textContent )
 );
 
-// For compatibility with other scripts that read from this global.
+// For compatibility with other scripts that read from this global, in particular wp-includes/js/wp-emoji.js (source file: js/_enqueues/wp/emoji.js).
 window._wpemojiSettings = settings;
 
 /**
@@ -422,17 +422,8 @@ new Promise( ( resolve ) => {
 			settings.supports.everythingExceptFlag &&
 			! settings.supports.flag;
 
-		// Sets DOMReady to false and assigns a ready function to settings.
-		settings.DOMReady = false;
-		settings.readyCallback = () => {
-			settings.DOMReady = true;
-		};
-	} )
-	.then( () => {
 		// When the browser can not render everything we need to load a polyfill.
 		if ( ! settings.supports.everything ) {
-			settings.readyCallback();
-
 			const src = settings.source || {};
 
 			if ( src.concatemoji ) {

src/js/_enqueues/wp/emoji.js

@@ -277,16 +277,7 @@
 			return twemoji.parse( object, params );
 		}
 
-		/**
-		 * Initialize our emoji support, and set up listeners.
-		 */
-		if ( settings ) {
-			if ( settings.DOMReady ) {
-				load();
-			} else {
-				settings.readyCallback = load;
-			}
-		}
+		load();
 
 		return {
 			parse: parse,

src/wp-includes/formatting.php

@@ -5912,7 +5912,11 @@ function print_emoji_detection_script() {
 
 	$printed = true;
 
-	_print_emoji_detection_script();
+	if ( did_action( 'wp_print_footer_scripts' ) ) {
+		_print_emoji_detection_script();
+	} else {
+		add_action( 'wp_print_footer_scripts', '_print_emoji_detection_script' );
+	}
 }
 
 /**