Add: Letter Combinations of a Phone Number

[awesee]

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[codecov-io]

Codecov Report

Merging #630 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #630   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files         185    186    +1     
  Lines        1866   1890   +24     
=====================================
+ Hits         1866   1890   +24

Impacted Files	Coverage Δ
...ne-number/letter_combinations_of_a_phone_number.go	100% <100%> (ø)

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