Add nextOccurrenceOf() and previousOccurrenceOf() methods

[dereuromark]

Summary

Adds two new methods to Chronos that solve the problem of constructing a relative date that always points to the next (or previous) future occurrence of a specific day and time.

Problem

As described in #406, PHP's relative date parsing with next only considers the date, not the time:

// If it's Tuesday 4pm:
new Chronos('Tue 12:00');       // Tuesday 12pm (in the past!)
new Chronos('next Tue 12:00');  // Next week Tuesday 12pm (always skips to next week)

There's no way to get "the next future occurrence of Tuesday 12pm" that returns today if it's before noon, or next week if it's after noon.

Solution

Two new methods:

• nextOccurrenceOf(int $dayOfWeek, int $hour, int $minute, int $second) - Returns the next future occurrence, considering both day and time
• previousOccurrenceOf(int $dayOfWeek, int $hour, int $minute, int $second) - Returns the most recent past occurrence

// If it's Tuesday 9am:
Chronos::now()->nextOccurrenceOf(Chronos::TUESDAY, 12, 0);
// → Today at 12pm (still in the future)

// If it's Tuesday 4pm:
Chronos::now()->nextOccurrenceOf(Chronos::TUESDAY, 12, 0);
// → Next Tuesday at 12pm (today's has passed)

Behavior at exact time

When the current time matches exactly, nextOccurrenceOf returns next week and previousOccurrenceOf returns last week (strict comparison, not inclusive).

Discussion Points

• Method naming: nextOccurrenceOf vs nextOrSame or something shorter?
• Should the time be at the exact second, or should it also consider greaterThanOrEquals?

Related to #406

Test plan

• Tests for same day before/after target time
• Tests for different day
• Tests for exact time edge case
• Tests for seconds parameter
• All existing tests pass (907 tests)
• Code style passes

[Bilge]

Seems OK to me.