Add Duplicate Character Counter in Javascript

[rzuckerm]

To request a new code snippet, please fill out the following:

Project name: Duplicate Character Counter
Project link: https://sampleprograms.io/projects/duplicate-character-counter
Language: Javascript

The project link must be a link to a project on the Sample Programs Website project list.
If you would like to add a new project to the Sample Programs Website,
please first make a pull request to the Sample Programs Website.

[georgioupanayiotis]

@rzuckerm can you assign me this ticket?