Emoji: Convert the emoji loader from an inline blocking script to a (deferred) script module.

This modernizes the emoji loader script by converting it from a blocking inline script with an IIFE to a script module. Using a script module improves the performance of the FCP and LCP metrics since it does not block the HTML parser. Since script modules are deferred and run immediately before `DOMContentLoaded`, the logic to wait until that event is also now removed. Additionally, since the script is loaded as a module, it has been modernized to use `const`, `let`, and arrow functions. The `sourceURL` comment is also added. See #63887.

The emoji settings are now passed via a separate `script` tag with a `type` of `application/json`, following the new "pull" paradigm introduced for exporting data from PHP to script modules. See #58873. The JSON data is also now encoded in a more resilient way according to #63851. When the `wp-emoji-loader.js` script module executes, it continues to populate the `window._wpemojiSettings` global for backwards-compatibility for any extensions that may be using it.

A new `uglify:emoji-loader` grunt task is added which ensures `wp-includes/js/wp-emoji-loader.js` is processed as a module and that top-level symbols are minified.

Follow-up to [60681].

Props westonruter, jonsurrell, adamsilverstein, peterwilsoncc.
See #63851, #63887.
Fixes #63842.


git-svn-id: https://develop.svn.wordpress.org/trunk@60899 602fd350-edb4-49c9-b593-d223f7449a82

Author: westonruter

Gruntfile.js

@@ -838,8 +838,17 @@ module.exports = function(grunt) {
 					'!**/*.min.js',
 					'!wp-admin/js/custom-header.js', // Why? We should minify this.
 					'!wp-admin/js/farbtastic.js',
+					'!wp-includes/js/wp-emoji-loader.js', // This is a module. See the emoji-loader task below.
 				]
 			},
+			'emoji-loader': {
+				options: {
+					module: true,
+					toplevel: true,
+				},
+				src: WORKING_DIR + 'wp-includes/js/wp-emoji-loader.js',
+				dest: WORKING_DIR + 'wp-includes/js/wp-emoji-loader.min.js',
+			},
 			'jquery-ui': {
 				options: {
 					// Preserve comments that start with a bang.
@@ -1549,6 +1558,7 @@ module.exports = function(grunt) {
 
 	grunt.registerTask( 'uglify:all', [
 		'uglify:core',
+		'uglify:emoji-loader',
 		'uglify:jquery-ui',
 		'uglify:imgareaselect',
 		'uglify:jqueryform',