Add Site Editor and PHP changes from Gutenberg 10.1 - 11.9

- First pass at adding the site editor from the Gutenberg plugin to
  wp-admin/site-editor.php.
- Adds miscellaneous PHP changes from Gutenberg 10.1 - 11.9.

Follows [52042].
See #54337.
Props youknowriad, aristath, hellofromtonya, gziolo.


git-svn-id: https://develop.svn.wordpress.org/trunk@52069 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/blocks/pattern/block.json

@@ -0,0 +1,17 @@
+{
+	"apiVersion": 2,
+	"name": "core/pattern",
+	"title": "Pattern",
+	"category": "design",
+	"description": "Show a block pattern.",
+	"supports": {
+		"html": false,
+		"inserter": false
+	},
+	"textdomain": "default",
+	"attributes": {
+		"slug": {
+			"type": "string"
+		}
+	}
+}