It’s nothing new with the block editor, it’s the same age-old way by putting the following code into your theme’s functions.php
:
add_action( 'init', 'wpse325327_add_excerpts_to_pages' );
function wpse325327_add_excerpts_to_pages() {
add_post_type_support( 'page', 'excerpt' );
}
Here’s my screenshot in a fresh WordPress 5.0.3 install: