Blocks: Introduce WP_Block_Type and WP_Block_Type_Registry classes.

These are the foundational classes allowing blocks to be registered and used throughout WordPress.

This commit also includes the `has_block()` and `has_blocks()` functions, which are required for unit testing these classes.

Merges [43742] from the 5.0 branch to trunk.

Props adamsilverstein, danielbachhuber, desrosj.
Fixes #45097.
See #45109.


git-svn-id: https://develop.svn.wordpress.org/trunk@44108 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/includes/bootstrap.php

@@ -133,6 +133,7 @@ require dirname( __FILE__ ) . '/exceptions.php';
 require dirname( __FILE__ ) . '/utils.php';
 require dirname( __FILE__ ) . '/spy-rest-server.php';
 require dirname( __FILE__ ) . '/class-wp-rest-test-search-handler.php';
+require dirname( __FILE__ ) . '/class-wp-fake-block-type.php';
 
 /**
  * A child class of the PHP test runner.

tests/phpunit/includes/class-wp-fake-block-type.php

@@ -0,0 +1,27 @@
+<?php
+/**
+ * WP_Fake_Block_Type for testing
+ *
+ * @package WordPress
+ * @subpackage Blocks
+ * @since 5.0.0
+ */
+
+/**
+ * Test class extending WP_Block_Type
+ *
+ * @since 5.0.0
+ */
+class WP_Fake_Block_Type extends WP_Block_Type {
+
+	/**
+	 * Render the fake block.
+	 *
+	 * @param array  $attributes Optional. Block attributes. Default empty array.
+	 * @param string $content    Optional. Block content. Default empty string.
+	 * @return string Rendered block HTML.
+	 */
+	public function render( $attributes = array(), $content = '' ) {
+		return '<div>' . $content . '</div>';
+	}
+}