feat(ui5-calendar): expose shadow parts for YearRangePicker and CalendarHeader arrows

[hinzzx]

Previously there was no possible way of overstyling the header navigation buttons and the year range picker root container 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(calendar-header-arrow-button) {
    background: var(--sapButton_Background, #ffffff);
    color: var(--sapButton_TextColor, #0070f2);
    border: 1px solid var(--sapButton_BorderColor, #0070f2);
    border-radius: 4px;
    box-shadow: 0 1px 3px rgba(0, 0, 0, 0.12);
}

ui5-calendar::part(year-range-picker-root) {
    padding: 16px 20px;
    background: var(--sapObjectHeader_Background, #ffffff);
    border: 1px solid var(--sapList_BorderColor, #e5e5e5);
    border-radius: 6px;
    box-shadow: 0 2px 8px rgba(0, 0, 0, 0.08);
}

Example

Fixes: #12090