Added JSON writer

[kenkoooo]

No description provided.

[codecov-io]

Codecov Report

Merging #82 into master will decrease coverage by 0.51%.
The diff coverage is 21.42%.

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
- Coverage   75.89%   75.37%   -0.52%     
==========================================
  Files          60       61       +1     
  Lines        1319     1332      +13     
  Branches       96       94       -2     
==========================================
+ Hits         1001     1004       +3     
- Misses        312      322      +10     
  Partials        6        6

Impacted Files	Coverage Δ
.../scala/com/kenkoooo/atcoder/common/Configure.scala	100% <ø> (ø)	⬆️
...end/src/main/scala/com/kenkoooo/atcoder/Main.scala	0% <0%> (ø)	⬆️
...scala/com/kenkoooo/atcoder/common/JsonWriter.scala	100% <100%> (ø)

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