missing solutions

[kristakoch]

Some of the solutions to the challenges are missing.

The solutions that are missing are:

• async.js misses exercises 8,9,10
• callback.js misses 12 up to 24
• closures.js misses 8 up to 19

Not missing anything:

• functional.js doesn't miss anything
• oop.js doesn't miss anything

[gildub]

Solutions for closures are available with #6

[hendridg]

Hi @kristakoch,

I want to contribute to the solution for challenge closures.js 🙏

[g-savitha]

I would like to contribute to callbacks solutions.

[g-savitha]

Hey @kristakoch I have added complete solutions for callbacks, please check #20