Standardize file names in packages/jest-diff

[andrearosr]

Summary

This is part of #4969 in follow-up to #6780.
It standardizes file names in packages/jest-diff under Facebook naming conventions.

Test plan

Running yarn run clean-all followed by yarn and yarn test is not completely clean on my machine, but it yields the same results in this branch and master

[SimenB]

Thanks! Hopefully CI triggers at some point, so we can verify this and land it.

In the meantime, mind updating the changelog? Under chore is fine 🙂

[codecov-io]

Codecov Report

Merging #7238 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #7238   +/-   ##
=======================================
  Coverage   66.55%   66.55%           
=======================================
  Files         237      237           
  Lines        9317     9317           
  Branches        3        3           
=======================================
  Hits         6201     6201           
  Misses       3115     3115           
  Partials        1        1

Impacted Files	Coverage Δ
...kages/jest-circus/src/format_node_assert_errors.js	16.27% <ø> (ø)	⬆️
packages/jest-diff/src/diffStrings.js	100% <ø> (ø)
packages/jest-diff/src/index.js	82.5% <ø> (ø)	⬆️
packages/jest-jasmine2/src/assert_support.js	0% <ø> (ø)	⬆️

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[github-actions]

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