https://www.codewars.com/kata/frequency-sequence/
Return an output string that translates an input string s/$s by replacing each character in s/$s with a number representing the number of times that character occurs in s/$s and separating each number with the character(s) sep/$sep.
Example:
freqSeq('hello world', '-'); // => '1-1-3-3-2-1-1-2-1-3-1'
freqSeq('^^^**$', 'x'); // => '3x3x3x2x2x1'
freqSeq('19999999', ':'); // => '1:7:7:7:7:7:7:7'