Add isWeekend() and isWeekday() methods to DateTime classes

[xentixar]

This PR adds two new methods to DateTime and DateTimeImmutable classes:

• isWeekend(): Checks if the date falls on a weekend (Saturday or Sunday)
• isWeekday(): Checks if the date falls on a weekday (Monday through Friday)

The methods are also declared in DateTimeInterface, ensuring they're available across all implementing classes.

Implementation includes:

• Helper functions in timelib library
• Method implementations for DateTime and DateTimeImmutable
• Proper interface declarations
• Test cases for basic functionality and inheritance

These methods provide a convenient way to check if a date is a weekend or weekday without manually checking the day of week.

[kamil-tekiela]

Why do you consider the weekend to be Saturday and Sunday? Why not other days? How is this determined? see https://en.wikipedia.org/wiki/Workweek_and_weekend#History

[iluuu1994]

Given the feedback, this probably requires a discussion on the mailing list and/or an RFC. Marking accordingly.