chore: add arrow-body-style eslint rule

[SimenB]

Summary

Less bytes to read is good, right?

Test plan

Green CI

[SimenB]

markdown is not autofixable (although the markdown plugin got a PR today, so maybe soon! (eslint/markdown#97))

[codecov-io]

Codecov Report

Merging #6406 into master will increase coverage by 0.01%.
The diff coverage is 48.57%.

@@            Coverage Diff             @@
##           master    #6406      +/-   ##
==========================================
+ Coverage   63.48%   63.49%   +0.01%     
==========================================
  Files         227      227              
  Lines        8697     8689       -8     
  Branches        4        3       -1     
==========================================
- Hits         5521     5517       -4     
+ Misses       3175     3171       -4     
  Partials        1        1

Impacted Files	Coverage Δ
packages/jest-resolve/src/index.js	96.72% <ø> (ø)	⬆️
...es/jest-cli/src/lib/handle_deprecation_warnings.js	0% <ø> (ø)	⬆️
packages/jest-config/src/normalize.js	93.85% <ø> (ø)	⬆️
packages/jest-jasmine2/src/setup_jest_globals.js	0% <0%> (ø)	⬆️
packages/jest-cli/src/watch.js	79.02% <0%> (ø)	⬆️
packages/jest-watcher/src/jest_hooks.js	76.47% <0%> (ø)	⬆️
packages/jest-jasmine2/src/assert_support.js	0% <0%> (ø)	⬆️
.../jest-editor-support/src/parsers/babylon_parser.js	92.2% <100%> (ø)	⬆️
packages/jest-cli/src/lib/watch_plugins_helpers.js	94.44% <100%> (ø)	⬆️
...e/coverage-report/cached-duplicates/a/Identical.js	100% <100%> (ø)	⬆️
... and 24 more

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[github-actions]

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