fix(luxon): incorrect slot dates during DST changes

[jhnns]

This PR fixes incorrect slot dates during DST changes when using the Luxon localizer.

DateTime.set() works with "wall clock" time - it sets the time as you'd see it on a wall clock, regardless of DST changes. DateTime.plus() works with actual elapsed time - it adds the specified duration, taking DST changes into account.

I've created a small example to demonstrate the problem:

// Example using a DST transition in US (March 12, 2023, 2 AM -> 3 AM)
const beforeDST = DateTime.fromISO('2023-03-12T00:00:00', { zone: 'America/New_York' });

console.log('\n=== DateTime.set() during DST change ===');
// Using set() - maintains the wall clock time, adjusting for DST
console.log('After set({ hours: 1 }):', beforeDST.set({ hours: 1 }).toLocaleString(DateTime.DATETIME_FULL));
console.log('After set({ hours: 2 }):', beforeDST.set({ hours: 2 }).toLocaleString(DateTime.DATETIME_FULL));
console.log('After set({ hours: 3 }):', beforeDST.set({ hours: 3 }).toLocaleString(DateTime.DATETIME_FULL));

console.log('\n=== DateTime.plus() during DST change ===');
// Using plus() - adds actual hours, considering DST change
console.log('After plus({ hours: 1 }):', beforeDST.plus({ hours: 1 }).toLocaleString(DateTime.DATETIME_FULL));
console.log('After plus({ hours: 2 }):', beforeDST.plus({ hours: 2 }).toLocaleString(DateTime.DATETIME_FULL));
console.log('After plus({ hours: 3 }):', beforeDST.plus({ hours: 3 }).toLocaleString(DateTime.DATETIME_FULL));

=== DateTime.set() during DST change ===
After set({ hours: 1 }): 12 March 2023 at 01:00 GMT-5
After set({ hours: 2 }): 12 March 2023 at 03:00 GMT-4
After set({ hours: 3 }): 12 March 2023 at 03:00 GMT-4

=== DateTime.plus() during DST change ===
After plus({ hours: 1 }): 12 March 2023 at 01:00 GMT-5
After plus({ hours: 2 }): 12 March 2023 at 03:00 GMT-4
After plus({ hours: 3 }): 12 March 2023 at 04:00 GMT-4