add code for goldbach conjecture algorithms

[djtrack16]

I wrote two functions for the famous Goldbach's conjecture, which basically says that for every natural number greater than 2, it can be rewritten as the sum of two primes.

This is the draft of the PR. If it is OK so far, then I will write tests for all these functions to complete the PR.

Suggestions welcome.

I'm happy to use a built-in range function but I couldn't find a suitable one. I'm new to Typescript since one month, so sorry if my code isn't idiomatic.