Added Javascript Solution for Count Primes

[gkelley]

Pull Request Template

Description

Given an integer n, return the number of prime numbers that are strictly less than n.

Example 1: Input: n = 10 Output: 4 Explanation: There are 4 prime numbers less than 10, they are 2, 3, 5, 7. 

No dependencies.

Solution:

• Assume every number is prime and create an array of all the numbers up to n. Loop through all numbers up to n and when you find a prime, set all the multiples of that number to not prime.

Put check marks:

• Added problem & solution under correct topic.
• Specified Space & Time complexity.
• Specified difficulty level, tag & Note(if any).

How Has This Been Tested?

• Tested via Leetcode solver

Make sure all below guidelines are followed else PR will get Reject:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code so that it is easy to understand
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

[welcome]

I can tell this is your first pull request! Thank you I'm so honored. 🎉🎉🎉 I'll take a look at it ASAP!