Add block type exists command

features/block-type.feature

@@ -159,3 +159,18 @@ Feature: Block type commands
       --dynamic and --static are mutually exclusive
       """
     And the return code should be 1
+
+  @require-wp-5.0
+  Scenario: Check if a block type exists
+    Given a WP install
+
+    When I run `wp block type exists core/archives`
+    Then STDOUT should be:
+      """
+      Success: Block type 'core/archives' is registered.
+      """
+    And the return code should be 0
+
+    When I try `wp block type exists core/nonexistent`
+    Then STDOUT should be empty
+    And the return code should be 1

src/Block_Type_Command.php

@@ -253,6 +253,43 @@ public function get( $args, $assoc_args ) {
 		$formatter->display_item( $data );
 	}
 
+	/**
+	 * Checks whether a block type is registered.
+	 *
+	 * Exits with return code 0 if the block type exists, 1 if it does not.
+	 *
+	 * ## OPTIONS
+	 *
+	 * <name>
+	 * : The block type name, including namespace.
+	 *
+	 * ## EXAMPLES
+	 *
+	 *     # Check if a block type exists.
+	 *     $ wp block type exists core/paragraph
+	 *     Success: Block type 'core/paragraph' is registered.
+	 *
+	 *     # Check for a non-existent block type.
+	 *     $ wp block type exists core/nonexistent
+	 *     $ echo $?
+	 *     1
+	 *
+	 * @subcommand exists
+	 *
+	 * @param array $args       Positional arguments.
+	 * @param array $assoc_args Associative arguments.
+	 */
+	public function exists_( $args, $assoc_args ) {
+		$registry = WP_Block_Type_Registry::get_instance();
+		$name     = $args[0];
+
+		if ( $registry->is_registered( $name ) ) {
+			WP_CLI::success( "Block type '{$name}' is registered." );
+		} else {
+			WP_CLI::halt( 1 );
+		}
+	}
+
 	/**
 	 * Converts a WP_Block_Type object to an associative array.
 	 *