jest-diff: Remove the need to export splitLines0 function

[pedrottimark]

Summary

Let callers of diffLinesUnified and diffLinesUnified2 use split method, because it is a private implementation issue that ''.split('\n') is [''] instead of []

But callers of diffLinesRaw must decide whether [''] or [] has correct meaning for their applications

In README.md file, move information:

• from Edge cases of diffLinesUnified or diffStringsUnified
• to Edge case of diffLinesRaw

This is not a breaking change, because the export was never published

Test plan

In diff.tests.ts

• Added 8 tests to diff.test.ts to cover new conditions
• Added 2 tests to close previous gap in code coverage
• Edited 1 describe to replace obsolete firstOrLastEmptyLineReplacement with emptyFirstOrLastLinePlaceholder default empty string

[codecov-io]

Codecov Report

Merging #9151 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #9151      +/-   ##
==========================================
+ Coverage   64.77%   64.81%   +0.04%     
==========================================
  Files         277      277              
  Lines       11724    11730       +6     
  Branches     2879     2881       +2     
==========================================
+ Hits         7594     7603       +9     
+ Misses       3512     3511       -1     
+ Partials      618      616       -2

Impacted Files	Coverage Δ
packages/jest-diff/src/index.ts	81.48% <100%> (ø)	⬆️
packages/jest-diff/src/diffLines.ts	100% <100%> (+4.76%)	⬆️
packages/jest-snapshot/src/printSnapshot.ts	100% <100%> (ø)	⬆️
packages/jest-diff/src/printDiffs.ts	100% <100%> (ø)	⬆️
...s/jest-core/src/lib/handle_deprecation_warnings.ts	0% <0%> (ø)	⬆️
packages/jest-environment-node/src/index.ts	57.14% <0%> (+0.38%)	⬆️

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[SimenB]

wonderful 👍

[github-actions]

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