Allow disabling of specific dates in jqueryui date picker.

I would not want to support individual options; instead, I'd like to make it easier for people (aka developers) to define custom options to a webform element.

Attached is a POC (untested) approach that adds support to data-options="{}" the data-options would always be applied last. This could become a universal solution that allows developers to define element-specific custom JS options.

The approach you could take would be something like...

data-options="{'beforeShowDay': my_custom_before_show_data'}"

function. my_custom_before_show_data(date) {
   // Ideally, you could pull the dates from the input's 'data-disabled-dates.'
    const dates = ['2024-12-17', '2024-12-18' , '2024-12-19' ];

    const formatted = date.toISOString().split('T')[0]
    return [!dates.includes(formatted)];
};

New features will only be added to 6.3.x

Let's leave this open because I think #5 needs to happen for most webform element libraries.

@see #3493832: Allow custom data-options to be added to elements JavaScript options