Curriculum: promote two-fer to core (round 2)

[SleeplessByte]

• sync two-fer with the problem specifications, add the default argument instead of calling with ''.
• promote two-fer to core, right after hello-world

[tejasbubane]

@SleeplessByte Comment on slack:

[SleeplessByte]

@tejasbubane taken care of at slack :)