Add support for prettier as a JSON Formatter

[omusavi]

Adds JSON_PRETTIER formatter. Prettier is already being used for many other languages and formats such as YAML, and it supports JSON. Adding it here for consistency in JSON files.

Proposed Changes

1. Added prettier to JSON descriptor
2. Added some tests
3. Ran build to generate docs

Readiness Checklist

Author/Contributor

• If documentation is needed for this change, has that been included in this pull request

Reviewing Maintainer

• Label as breaking if this is a large fundamental change
• Label as either automation, bug, documentation, enhancement, infrastructure, or performance

[codecov-commenter]

Codecov Report

Merging #421 (7b49f95) into master (4652b2b) will increase coverage by 1.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #421      +/-   ##
==========================================
+ Coverage   86.62%   87.67%   +1.05%     
==========================================
  Files         123      124       +1     
  Lines        2915     2921       +6     
==========================================
+ Hits         2525     2561      +36     
+ Misses        390      360      -30     

Impacted Files	Coverage Δ
...ests/test_megalinter/linters/json_prettier_test.py	100.00% <100.00%> (ø)
megalinter/MegaLinter.py	90.62% <0.00%> (+2.43%)	⬆️
megalinter/reporters/UpdatedSourcesReporter.py	89.18% <0.00%> (+2.70%)	⬆️
...alinter/tests/test_megalinter/helpers/utilstest.py	99.12% <0.00%> (+9.64%)	⬆️

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[nvuillam]

Nice idea, thanks a lot for your contribution :)

[nvuillam]

great :)