[v3] Add prerequisites 2/6

[TomPradat]

⚠️ If you have issues / are unsure, create ONE PR PER EXERCISE so it's easier to merge them as we can have discussions per exercise. If you're certain, it's fine to do a single one.

The idea is to split #960 into smaller chunks.

Here is the list of practices exercises to deal with for this part :

~collatz-conjecture~

~triangle~

~clock~

~meetup~

~etl~

~hamming~

~raindrops~

~nucleotide-count~

scrabble-score

allergies

word-count

bank-account

~difference-of-squares~

perfect-numbers

~complex-numbers~

luhn

~prime-factors~

grains

pythagorean-triplet

palindrome-products

[TomPradat]

I'll work on this issue

[TomPradat]

Update, here is the list of exercises that still need to be addressed :

• scrabble-score

• allergies

• word-count

• bank-account

• perfect-numbers

• luhn

• grains

• pythagorean-triplet

• palindrome-products

I'll work on this :)

[SleeplessByte]

That's it I guess?!