Local installation and `script_path` option leads to an error

I think I found a bug related to a local installation when you try to set a custom language (but it looks to be a more global issue related to script_path option, it might be linked to #718).

Steps to reproduce:

1. get the zip file from https://timeline.knightlab.com/docs/instantiate-a-timeline.html#local_files and unzip it in a local folder
2. copy-past that minimal example to reproduce the issue:

<link title="timeline-styles" rel="stylesheet" href="/timeline3/css/timeline.css">
<script src="/timeline3/js/timeline.js"></script>

<div id='timeline-embed' style="width: 100%; height: 100vh"></div>


<script id="timeline-data" type="application/json">
  {
    "title": {
        "text": {
          "headline": "A bug report",
          "text": "About script_path and local files."
        }
    },
    "events": [
      {
        "start_date": {
          "day": "25",
          "month": "05",
          "year": "2022"
        },
        "text": {
          "headline": "Creation",
          "text": "With a minimal scenario."
        }
      }
    ]
  }

</script>


<script type="text/javascript">
    const timeline_json = JSON.parse(
      document.getElementById('timeline-data').textContent
    )
    const options = {
      // This is the value that raises an error.
      script_path: "/timeline3/js/",
      // To make sure it gets used.
      language: "fr"
    }
    window.timeline = new TL.Timeline('timeline-embed', timeline_json, options)
</script>

3. check the console of your browser (Firefox 100.0.1 in my case but tested with Chromium too), there is an error Uncaught TypeError: URL constructor: /timeline3/js/ is not a valid URL. even if you serve that folder.

If I switch the script_path option to a full local URL (let's say http://127.0.0.1:8000/timeline3/js/ it does solve the problem but it's not really portable 😄).

That line looks to be responsible of the issue: https://github.com/NUKnightLab/TimelineJS3/blob/master/src/js/timeline/Timeline.js#L219 and there is probably something to fix on that line too: https://github.com/NUKnightLab/TimelineJS3/blob/master/src/js/timeline/Timeline.js#L232

Please let me know if I missed something, I'm new to the project. I'm willing to contribute with a PR if there is some guidance from a more experimented person 🤗

Start with the local installation example and inspect src/js/timeline/Timeline.js at the reported lines 219 and 232. Reproduce the error with a relative script_path and language set to fr, then verify that the local timeline loads without the URL constructor error while retaining support for a full local URL.