Add initial support for eslint.

[robertoraggi]

Add initial support for eslint.

This change adds an eslintrc configuration file with prettier
integration and common checks for typescript and standard js.
The npm script `eslint` can be used to check the files, e.g.

```
   $ yarn eslint
```

For now, the rules that the current codebase of harp.gl
is not following are disabled.

[codecov]

Codecov Report

Merging #1637 into master will increase coverage by 0.20%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1637      +/-   ##
==========================================
+ Coverage   62.33%   62.53%   +0.20%     
==========================================
  Files         274      274              
  Lines       24749    24749              
  Branches     5572     5572              
==========================================
+ Hits        15428    15478      +50     
+ Misses       9321     9271      -50     

Impacted Files	Coverage Δ
...here/harp-mapview/lib/text/TextElementsRenderer.ts	69.56% <0.00%> (+1.08%)	⬆️
@here/harp-mapview/lib/MapView.ts	65.69% <0.00%> (+1.37%)	⬆️
...ere/harp-mapview/lib/composing/LowResRenderPass.ts	44.23% <0.00%> (+3.84%)	⬆️
@here/harp-mapview/lib/CameraMovementDetector.ts	90.14% <0.00%> (+28.16%)	⬆️

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