feat(ui5-calendar): expose shadow parts for month-picker and year-picker

[hinzzx]

Previously there was no possible way of overstyling the header buttons, the month picker view, and the year picker's buttons of the ui5-calendar web component.

With this change we are introducing CSS Shadow Parts, which allows overstyling of those calendar parts.

ui5-calendar::part(month-picker-root) {
			background: linear-gradient(135deg, #e3f2fd, #bbdefb);
			border: 2px solid #2196f3;
			border-radius: 8px;
		}

Example: