SpoilerAlert/includes/SpoilerAlertHooks.php

<?php
class SpoilerAlertHooks {
    public static function onParserFirstCallInit( Parser $parser ) {
        $parser->setHook( 'spoiler', [ self::class, 'renderSpoiler' ] );
        return true;
    }

    public static function renderSpoiler( $input, array $args, Parser $parser, PPFrame $frame ) {
        $output = $parser->recursiveTagParse( $input, $frame );
        return "<span class='spoiler' onclick='this.classList.toggle(\"revealed\")'>" . htmlspecialchars( $output ) . "</span>";
    }

    public static function onBeforePageDisplay( OutputPage &$out, Skin &$skin ) {
        $out->addModuleStyles( ['ext.spoileralert'] );
        return true;
    }
}