feat(enumerables): add motzkin number sequence

[edelveart]

• [x] Adds Motzkin Numbers sequence (via Enumerator class).

• [ ] Add sequence to Wiki.

[edelveart]

Infinite sequences (Enumerator | Lazy) based on the book Figurate Numbers (2012) by Elena Deza and Michel Deza.

• [x] Add sequences of figurate numbers in the plane
• [x] Add sequences of figurate numbers in space

[amiika]

Great work. Thanks!