fix: enable accessibility for charts

[tulioag]

Migrated PR #558 to the repo so tests can run.

related to #474

Workaround mentioned (highcharts/highcharts#13482) was already fixed in 8.1.1 (https://www.highcharts.com/blog/changelog/).

Vaadin-charts already depend on 8.1.2.

[web-padawan]

The PR needs the following to make tests pass:

1. Add the CSS for markers based on Improved fix for #13482, forced markers in styled mode. highcharts/highcharts#13563

.highcharts-a11y-markers-hidden .highcharts-point:not(.highcharts-point-hover):not(.highcharts-a11y-marker-visible),
.highcharts-a11y-marker-hidden {
    opacity: 0;
}

2. Update markersVisible helper to take opacity into account:

    function markersVisible(container) {
      return Array.from(container.querySelectorAll('.highcharts-markers')).every((g) => {
        return Boolean(g.firstChild && getComputedStyle(g.firstChild).opacity !== '0');
      });
    }

[tulioag]

@web-padawan I made the changes you requested