Bootstrap/Load: Make handling the /favicon.ico requests more flexible.

Previously, `wp_favicon_request()` was introduced in [13205] to avoid a performance hit of serving a full 404 page on every favicon request.

While working as intended, that implementation did not provide a way for theme or plugin authors to manage the behavior of favicon requests.

This changeset implements the following logic (only applied if WordPress is installed in the root directory):

* If there is a Site Icon set in Customizer, redirect `/favicon.ico` requests to that icon.
* Otherwise, use the WordPress logo as a default icon.
* If a physical `/favicon.ico` file exists, do nothing, let the server handle the request.

Handling `/favicon.ico` is now more consistent with handling `/robots.txt` requests.

New functions and hooks:

* Introduce `is_favicon()` conditional tag to complement `is_robots()`.
* Introduce `do_favicon` action to complement `do_robots` and use it in template loader.
* Introduce `do_favicon()` function, hooked to the above action by default, to complement `do_robots()`.
* Introduce `do_faviconico` action to complement `do_robotstxt`, for plugins to override the default behavior.
* Mark `wp_favicon_request()` as deprecated in favor of `do_favicon()`.

Props jonoaldersonwp, birgire, joostdevalk, mukesh27, SergeyBiryukov.
Fixes #47398.

git-svn-id: https://develop.svn.wordpress.org/trunk@47018 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/query.php

@@ -623,7 +623,7 @@ function is_preview() {
 }
 
 /**
- * Is the query for the robots file?
+ * Is the query for the robots.txt file?
  *
  * @since 2.1.0
  *
@@ -642,6 +642,26 @@ function is_robots() {
 	return $wp_query->is_robots();
 }
 
+/**
+ * Is the query for the favicon.ico file?
+ *
+ * @since 5.4.0
+ *
+ * @global WP_Query $wp_query WordPress Query object.
+ *
+ * @return bool
+ */
+function is_favicon() {
+	global $wp_query;
+
+	if ( ! isset( $wp_query ) ) {
+		_doing_it_wrong( __FUNCTION__, __( 'Conditional query tags do not work before the query is run. Before then, they always return false.' ), '3.1.0' );
+		return false;
+	}
+
+	return $wp_query->is_favicon();
+}
+
 /**
  * Determines whether the query is for a search.
  *