Add package: autop

[aduth]

Related: WordPress/gutenberg#4005

This pull request seeks to introduce a new autop package with two named exports: autop and removep. Originally these were extracted verbatim from core's editor.js. However, it was found that the JavaScript implementation of autop was lacking in many ways when I sought to port the equivalent PHP tests for wpautop. You can see the failing skipped tests in f31289b.

Instead, I re- ported the function directly from the PHP implementation as faithfully as possible. With the reimplementation in 4846972, all test cases ported from the PHPUnit tests pass.

Testing instructions:

Ensure unit tests pass:

npm test

[codecov]

Codecov Report

Merging #53 into master will decrease coverage by 19%.
The diff coverage is 59.47%.

@@             Coverage Diff             @@
##           master      #53       +/-   ##
===========================================
- Coverage   97.36%   78.36%   -19.01%     
===========================================
  Files          17       18        +1     
  Lines         152      305      +153     
  Branches       43       67       +24     
===========================================
+ Hits          148      239       +91     
- Misses          4       55       +51     
- Partials        0       11       +11

Impacted Files	Coverage Δ
packages/autop/src/index.js	59.47% <59.47%> (ø)

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[azaozz]

However, it was found that the JavaScript implementation of autop was lacking in many ways

Not exactly :)
The JS autop is meant to work together with TinyMCE and the browser auto-correct: white space doesn't matter, closing </p> tags are optional, TinyMCE converts text that is directly in the body to paragraphs, etc. The three combined have generally less edge case errors than the PHP autop. Some PHP tests were probably failing as they look also at white space between tags.

Nevertheless this version should work properly, a bit slower perhaps. Did some "manual" testing in the current editor and didn't see regressions except one case where extra empty paragraphs were added (but couldn't repeat/confirm).