You can register a simple block without a build step. Load the script in the editor with the enqueue_block_editor_assets hook and declare the WordPress packages as dependencies.

functions.php

function lotgrafix_enqueue_block_editor_script() {
    wp_enqueue_script(
        'lotgrafix-block',
        get_template_directory_uri() . '/js/block.js',
        array( 'wp-blocks', 'wp-element', 'wp-block-editor' ),
        '1.0.0',
        true
    );
}
add_action( 'enqueue_block_editor_assets', 'lotgrafix_enqueue_block_editor_script' );

block.js

( function ( blocks, element, blockEditor ) {
    var el = element.createElement;
    var useBlockProps = blockEditor.useBlockProps;

    blocks.registerBlockType( 'lotgrafix/hello-block', {
        title: 'Lotgrafix Hello Block',
        icon: 'smiley',
        category: 'widgets',
        edit: function () {
            return el( 'p', useBlockProps(), 'Hello from the editor!' );
        },
        save: function () {
            return el( 'p', useBlockProps.save(), 'Hello from the frontend!' );
        },
    } );
} )( window.wp.blocks, window.wp.element, window.wp.blockEditor );