ENH add support for expanded values on Time/DateField

[lekoala]

Description

Currently, partial values in date/time field will results in null values. This is a bit annoying for time value (input 18:--:-- will be null) but is even more obvious on datetime field (inputing the date without the time results in null)

Manual testing steps

Only input partial values, it still submit properly

Issues

• TimeField should deal better with partial times #11172

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[lekoala]

Still work in progress

the basic idea would be to add some kind of common helper

    private function expandValue(?string $value, IntlDateFormatter $formatter): int|false
    {
        $zeroFormat = $formatter->format(strtotime(date('Y-01-01 00:00:00')));
        $expectedLength = strlen($zeroFormat);
        if (strlen($value) >= $expectedLength) {
            return false;
        }
        $expandedValue = $value . substr($zeroFormat, strlen($value));
        return $formatter->parse($expandedValue);
    }

i realize that there are many cases where you can setValue, setSubmittedValue, convert from frontend to internal, tidy internals, ... it's not always super consistent and basically i'd like partial values to work everywhere without too much issues

[GuySartorelli]

@lekoala Are you still keen to work on this? It seems like it'd be a really useful feature.