Adds SlotFills/meta box example (#153)

* Adds SlotFills/meta box example

* Address JS formatting issues

* Removes unnecessary call to register_block_type

07-slotfills-esnext/.eslintrc

@@ -0,0 +1,3 @@
+{
+    "extends":  [ "plugin:@wordpress/eslint-plugin/recommended" ]
+}

07-slotfills-esnext/build/index.asset.php

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+<?php return array('dependencies' => array('wp-components', 'wp-core-data', 'wp-data', 'wp-edit-post', 'wp-element', 'wp-plugins', 'wp-polyfill'), 'version' => '24e3bebed355ba21de523036f0729a6b');

07-slotfills-esnext/index.php

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+<?php
+/**
+ * Plugin Name: Gutenberg Examples SlotFills ESNext
+ * Plugin URI: https://github.com/WordPress/gutenberg-examples
+ * Description: This is a plugin demonstrating how to use SlotFills in the Gutenberg editor.
+ * Version: 1.0.0
+ * Author: the Gutenberg Team
+ *
+ * @package gutenberg-examples
+ */
+
+defined( 'ABSPATH' ) || exit;
+
+/**
+ * Registers all block assets so that they can be enqueued through Gutenberg in
+ * the corresponding context.
+ */
+function gutenberg_examples_07_esnext_register_dependencies() {
+	// Automatically load dependencies and version.
+	$asset_file = include plugin_dir_path( __FILE__ ) . 'build/index.asset.php';
+
+	wp_register_script(
+		'gutenberg-examples-07-esnext',
+		plugins_url( 'build/index.js', __FILE__ ),
+		$asset_file['dependencies'],
+		$asset_file['version'],
+		true
+	);
+
+	// Register the post meta field the meta box will save to.
+	register_post_meta(
+		'post',
+		'example_meta_field',
+		array(
+			'show_in_rest' => true,
+			'single'       => true,
+			'type'         => 'string',
+		)
+	);
+}
+add_action( 'init', 'gutenberg_examples_07_esnext_register_dependencies' );
+
+/**
+ * Enqueue block editor assets for this example.
+ */
+function gutenberg_examples_07_esnext_enqueue_assets() {
+	wp_enqueue_script( 'gutenberg-examples-07-esnext' );
+}
+add_action( 'enqueue_block_editor_assets', 'gutenberg_examples_07_esnext_enqueue_assets' );