Add (optional) excalidraw custom asset path

[gerlingsm]

Replace pull request #1652 and fix issue #1651

[ggrossetie]

@gerlingsm It seems that using an empty value is the best of both worlds since it won't load assets from a CDN server side but it will use the CDN on client side (for fonts):

<svg xmlns="http://www.w3.org/2000/svg" version="1.1" viewBox="0 0 577 515" width="577" height="515">
  <!-- svg-source:excalidraw -->
  <defs>
    <style class="style-fonts">
      @font-face {
        font-family: &quot;Virgil&quot;;
        src: url(&quot;https://unpkg.com/@excalidraw/[email protected]/dist/excalidraw-assets/Virgil.woff2&quot;);
      }
      @font-face {
        font-family: &quot;Cascadia&quot;;
        src: url(&quot;https://unpkg.com/@excalidraw/[email protected]/dist/excalidraw-assets/Cascadia.woff2&quot;);
      }
    </style>
  </defs>