Embeds: Conditionally enqueue wp-embed only if needed and send ready message in case script loads after post embed windows.

* Prevent loading `wp-embed` script unconditionally on every page in favor of conditionally enqueueing when a post embed is detected. The `wp-embed` script is also explicitly marked as being in the footer group. Sites which currently disable post embed scripts from being enqueued via `remove_action( 'wp_head', 'wp_oembed_add_host_js' )` will continue to do so.
* Send a `ready` message from the host page to each post embed window in case the `iframe` loads before the `wp-embed` script does. When the `ready` message is received by the post embed window, it sends the same `height` message as it sends when it loads.
* Eliminate use of `grunt-include` to inject emoji script and the post embed script. Instead obtain the script contents via `file_get_contents()` (as is done elsewhere in core) and utilize `wp_print_inline_script_tag()`/`wp_get_inline_script_tag()` to construct out the script. This simplifies the logic and allows the running of src without `SCRIPT_DEBUG` enabled.
* For the embed code that users are provided to copy for embedding outside of WP, add the `secret` on the `blockquote` and `iframe`. This ensures the `blockquote` will be hidden when the `iframe` loads. The embed code in question is accessed here via `get_post_embed_html()`.

Props westonruter, swissspidy, pento, flixos90, ocean90.
Fixes #44632, #44306.


git-svn-id: https://develop.svn.wordpress.org/trunk@52132 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/script-loader.php

@@ -1247,7 +1247,7 @@ function wp_default_scripts( $scripts ) {
 		)
 	);
 
-	$scripts->add( 'wp-embed', "/wp-includes/js/wp-embed$suffix.js" );
+	$scripts->add( 'wp-embed', "/wp-includes/js/wp-embed$suffix.js", array(), false, 1 );
 
 	// To enqueue media-views or media-editor, call wp_enqueue_media().
 	// Both rely on numerous settings, styles, and templates to operate correctly.