Reflect internal states in Date Picker's month calendar as part names

[rolfsmeds]

Describe your motivation

The following states of the date part in vaadin-month-calendar are not targetable with the ::part() selector:

• today
• selected
• disabled
• focused

Describe the solution you'd like

Reflect these as dynamically applied part names.

Example: selected date in month calendar: Before: <td selected part="date">30</td> After: <td selected part="date selected">30</td> or <td selected part="date date-selected">30</td>, depending on how we choose to name state parts.

Describe alternatives you've considered

No response

Additional context

No response