feat: add alt-text for maidr plot

[jooyoungseo]

This pull request adds alt-text to the maidr plot by setting the aria-label and title attributes for the chart. Additionally, the img role is set for the chart. This ensures that the chart is accessible to users who rely on screen readers and just mouse.

fix issue #567.

[jooyoungseo]

@ellvix I wanted to add 'img' role to the constants.chart, but it did not work. I think it is because the constants.chart.setAttribute() that I used was not fired until the chart is initiated when focused. However, I need to add this attribute as soon as the page is loaded and the chart is loaded regardless of whether the maidr chart gets focused or not. Because screen reader users and sighted users need to hear and see the how-to-activate and use maidr instruction via aria-label and title tooltip. Users need to be provided the instruction before they tab-focus in the maidr plot.