A11y improvements

[remischwartz]

Hi!

Thank you for your amazing library! We used it to build our accessible web components design system.

We received some feedback from our a11y consultant, and he identified some improvement opportunities:

• Add an aria-label attribute to the th elements. The abbr attribute is not announced by all screen readers.
• Only change the value of the hidden span in wc-datepicker__header when the selected value changes (currently, it changes when users navigate through the dates with the keyboard).
• In range mode, change the value of the hidden span in wc-datepicker__header to express the selected range (e.g., “June 21, 2024 to June 25, 2024”).
• Add a title attribute to the month and year selection fields (same value as the aria-label).
• When using disableDate, prevent the disabled dates from being focused.

Do you think these changes could be implemented? I can open PRs if you prefer.