[create-block] Add ABSPATH check

[KevinBatdorf]

What?

This PR adds the direct file access call to the create-block stub files. This impacts users that run npx @wordpress/create-block my-plugin

Why?

To prevent direct access to the file by default.

(I was also reminded by the plugin team a recent submission of mine was missing it)

How?

Added the following to the /plugin-name.php stub files

if ( ! defined( 'ABSPATH' ) ) {
	exit; // Exit if accessed directly.
}

I know there are many ways to write this code, so I arbitrarily picked one I've seen in a core project (performance plugin)

(edit) Technically I think defined( 'ABSPATH' ) or exit; would actually be the fastest given the the NOT opcode call is not there, but arguably it's less readable code for newer devs.

Alternatives in core:

https://github.com/WordPress/gutenberg/blob/trunk/lib/demo.php#L8

if ( ! defined( 'ABSPATH' ) ) {
	die( 'Silence is golden.' );
}

https://github.com/WordPress/gutenberg/blob/trunk/lib/demo.php#L8

if ( ! defined( 'ABSPATH' ) ) {
	die();
}

https://github.com/WordPress/WordPress/blob/master/wp-includes/rss-functions.php#L9

if ( ! defined( 'ABSPATH' ) ) {
	exit();
}

https://github.com/WordPress/WordPress/blob/master/wp-admin/admin-footer.php#L10

if ( ! defined( 'ABSPATH' ) ) {
	die( '-1' );
}

Health Check:

https://github.com/WordPress/health-check/blob/trunk/pages/site-status.php#L9

if ( ! defined( 'ABSPATH' ) ) {
	die( 'We\'re sorry, but you can not directly access this file.' );
}

Performance plugin:

https://github.com/WordPress/performance/blob/trunk/modules/images/webp-uploads/can-load.php#L9

if ( ! defined( 'ABSPATH' ) ) {
	exit; // Exit if accessed directly.
}

Some others I've seen in the wild

if ( ! defined( 'ABSPATH' ) ) exit; // Exit if accessed directly
defined('ABSPATH') or die;
defined( 'ABSPATH' ) || exit;

[github-actions]

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[gziolo]

It makes perfect sense. Thank you so much for improving the templates based on your experience to meet best practices ❤️