Introduce efmt code formatter

[sile]

I recently released v0.1.0 of efmt which is an Erlang code formatter.
This PR introduces the formatter to jsone for dogfooding.

[codecov-commenter]

Codecov Report

Merging #71 (153dd84) into master (31e9f0e) will increase coverage by 0.09%.
The diff coverage is 90.90%.

❗ Current head 153dd84 differs from pull request most recent head 7a5e359. Consider uploading reports for the commit 7a5e359 to get more accurate results

@@            Coverage Diff             @@
##           master      #71      +/-   ##
==========================================
+ Coverage   91.91%   92.01%   +0.09%     
==========================================
  Files           4        4              
  Lines         334      338       +4     
==========================================
+ Hits          307      311       +4     
  Misses         27       27              

Impacted Files	Coverage Δ
src/jsone.erl	82.60% <0.00%> (ø)
src/jsone_encode.erl	86.53% <83.33%> (+0.08%)	⬆️
src/jsone_decode.erl	98.44% <100.00%> (+0.03%)	⬆️
src/jsone_inet.erl	100.00% <100.00%> (ø)

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