refactor: modernize javascript and remove div

[TendTo]

First of all, I'm really grateful for the script you provided. I tried to achieve something similar on my own but I always ran into some issues with errors and strange rendering sizes. This script works like a charm.

Just for the sake of it, I tried to modernize it a bit, using arrow functions and string interpolation.

Also, I noticed you created an additional div containing the div with the mermaid svg. I think it is safe to remove the external div.

<!-- Current structure -->
<div>
  <div class="mermaid" data-processed="true">
    <svg>
        <!-- graph -->
    </svg>
  <div>
<div>

<!-- New structure -->
<div class="mermaid" data-processed="true">
  <svg>
      <!-- graph -->
  </svg>
<div>

Let me know what do you think, if you have any suggestion or if you prefer things as they are now.