[FR] Add BEFORE_RENDER_ELEMENT event to ElementHelper

[leevigraham]

I would like to override the _parials template rendering logic with my own logic while retaining the developer ergonomics of .render.

Something like:

if (Event::hasHandlers(static::class, self::EVENT_BEFORE_RENDER_ELEMENT)) {
    $output = null;
    $event = new BeforeRenderElementEvent([
        'element' => $element,
        'variables' => $variables,
        'templates' => $templates,
        'output' => $output,
    ]);
    Event::trigger(static::class, self::EVENT_BEFORE_RENDER_ELEMENT, $event);
    return $event->output;
}