register-block-style-with-handle.php

<?php
function register_uppercase_heading() {

	wp_register_style( 'block-style-uppercase-heading', get_template_directory_uri() . '/block-style-uppercase-heading.css' );

	register_block_style(
		'core/heading',
		array(
			'name'         => 'uppercase-heading',
			'label'        => 'Uppercase',
			'style_handle' => 'block-style-uppercase-heading',
		)
	);

}
add_action( 'after_setup_theme' , 'register_uppercase_heading' );
?>