Port run-length-encoding exercise from JavaScript track

As part of the job to make the track compatible with Nextercism, we identified there are missing exercises in this track that are present in the JavaScript track. One of them is run-length-encoding.

The task would be to add the run-length-encoding exercise to this track, copying files from the JavaScript track and adapting the code to the ECMAScript 6 standard.

Basic steps would include:

1. Copy all files from <javascript repo>/exercises/run-length-encoding (https://github.com/exercism/javascript) to <ecmascript repo>/exercises/run-length-encoding (https://github.com/exercism/ecmascript)
2. Copy the main package.json file (it’s in the root folder of the ecmascript repo https://github.com/exercism/ecmascript/blob/master/package.json) to the <ecmascript repo>/exercises/run-length-encoding folder
3. Adapt the code in run-length-encoding.spec.js file to use ECMAScript 6 features (import modules, arrow functions, const and/or let, any other feature you think will make the code better). Additionally, the JavaScript track uses Jasmine where the ECMAScript track uses Jest. So, it and xit must be translated to test and xtest. Feel free to adapt example.js as well, just for fun (as long as tests pass).
4. In the README.md of the exercise, replace sections from Setup and below (Setup, Making the test suite pass, Source and Submitting incomplete solutions) with similar sections from any ECMAScript exercise, for example accumulate README. They’ll include Setup, Requirements, Making the test suite pass, Source and Submitting incomplete solutions.
5. Copy some exercise information from JavaScript config.json file to the ECMAScript config.json file. This information is an object in the exercises array property with a slug property equals to run-length-encoding. Copy that object and place it in the same (or most similar) position in the ECMAScript config.json file. Both config.json files are in the root folder of the repo.

@exercism/ecmascripts - Do you miss something here?

[matthewmorgan]

@rchavarria as I mentioned in the JS track, we also need to change the test suite methods to use Jest here instead of Jasmine, so it becomes test and xit becomes xtest.

Thanks for doing this!

[rchavarria]

Yep, it’s true. Thanks for helping on keeping issue list clean 👍

Closed by #335