add toggleElementEnabled member to DomUtility

[BryanWilhite]

    static toggleElementEnabled(element: HTMLElement | null): void {
        if (!element) { return; }

        element.toggleAttribute('disabled');
    }

https://github.com/BryanWilhite/Songhay.Player.ProgressiveAudio/blob/9ae33420745c54611cf4aabb40f6d0119245e520/Songhay.StudioFloor.Client/src/progressive-audio-utility.ts#L107

[BryanWilhite]

from my Obsidian notes:

forgetting the basics on the DOM…

Calling element.toggleAttribute(disabled) also sets the element.disabled property:

There is no need to set the element.disabled property explicitly. Additionally, because not all elements have a disabled property, slapping on an attribute is more ‘flexible.’