Addresses the bug bigskysoftware/htmx#2599 ... raised against the original repo.

Description

currently the head-support feature starts doing its merging based off an htmx:afterSwap event. This means that any script being loaded into the DOM from the htmxSwap that wants to run some initialisation code may run after the htmx:afterHeadMerge event has triggered, and never see it.

An example for this would be if you want to return a ChartJS from an endpoint along with the script to display the chart.

    <head>
      <script src="https://cdn.jsdelivr.net/npm/chart.js"></script>
    </head>
    <canvas id="myChart"></canvas>
    <script>
      htmx.on('htmx:afterHeadMerge', function () {
        (async () => {
          while (!Chart) {
            await new Promise((resolve) => setTimeout(resolve, 100));
          }
          var ctx = document.getElementById('myChart').getContext('2d');
          var myChart = new Chart(ctx, { <CONFIG> });
        })();
      });
    </script>

The polling is necessary to check that the Chart class has been declared (i.e. the new head script has loaded)
(and yes this could do with a timeout so it doesn't try for ever, but this is the basic solution)
However, this code won't work every time if we have htmx:afterSwap in head-support.js as the trigger.

Changing htmx:afterSwap to htmx:afterSettle ensures the DOM has settled (i.e. our event listener has been registered) before the head swap functionality is performed and our code works as expected.

Testing

This was tested with the above code by calling this code on a lazy-loaded hx-post.
The race condition seems to be gone.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded