AddToConfig.php

<?php

\SilverStripe\View\Parsers\ShortcodeParser::get('default')->unregister('embed');
\SilverStripe\View\Parsers\ShortcodeParser::get('default')->register(
    'embed',
    function ($arguments, $content = null, $parser = null, $tagName) {
        return \Your\Namespace\Here\MyEmbedShortcodeProvider::handle_shortcode(
            $arguments,
            $content,
            $parser,
            $tagName
        );
    }
);