Site Editor: Add 'path' query arg to Appearance > Template Parts menu.

To display a list of block-based template parts for classic themes, this changeset adds a `path` query arg to the Appearance > Template Parts menu URL. This change is necessary due to recent changes to Site Editor's client-side routing.

Reference:
* [WordPress/gutenberg#48343 Gutenberg PR 48343]
* [WordPress/gutenberg#42729 Gutenberg PR 42729]

Follow-up to [55392], [54176].

Props mamaduka, audrasjb, fabiankaegy, hellofromTonya, kevin940726, mukesh27, youknowriad.
Fixes #57819.

git-svn-id: https://develop.svn.wordpress.org/trunk@55435 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/menu.php

@@ -210,7 +210,7 @@
 	$submenu['themes.php'][6] = array(
 		__( 'Template Parts' ),
 		'edit_theme_options',
-		'site-editor.php?postType=wp_template_part',
+		'site-editor.php?postType=wp_template_part&path=/wp_template_part/all',
 	);
 }