refactor: use prettier, switch to 2 indent, no-semi

[kt3k]

This PR introduce prettier formatter and switches from 4 space indent to 2 space indent, and also switches from semicolon style to no-semicolon style.

(After this change I will remove jshint and introduces eslint instead.)

closes #2763

[codecov-io]

Codecov Report

Merging #2766 into master will decrease coverage by 0.88%.
The diff coverage is 49.13%.

@@            Coverage Diff             @@
##           master    #2766      +/-   ##
==========================================
- Coverage   82.91%   82.03%   -0.89%     
==========================================
  Files          60       60              
  Lines        4793     5159     +366     
==========================================
+ Hits         3974     4232     +258     
- Misses        819      927     +108

Impacted Files	Coverage Δ
src/arc.js	76.14% <ø> (+1.06%)	⬆️
src/util.js	98.52% <ø> (+0.02%)	⬆️
src/zoom.js	70% <ø> (-0.13%)	⬇️
src/stanford.js	93.5% <ø> (+0.08%)	⬆️
src/grid.js	74.34% <ø> (-0.47%)	⬇️
src/colorscale.js	82.05% <ø> (ø)	⬆️
src/config.js	95.83% <ø> (ø)	⬆️
src/tooltip.js	81.45% <ø> (-0.67%)	⬇️
src/data.load.js	60.37% <ø> (-3.63%)	⬇️
src/subchart.js	84.76% <ø> (-2.74%)	⬇️
... and 86 more

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[kt3k]

@masayuki0812 Please take a look if you have time.

[kt3k]

@masayuki0812 I believe this is a sensible change and this style conforms better to the modern trends of javascript ecosystem. Please let me know if you have a different opinion.