i18n: Extract concatenated strings of translate functions

[aduth]

Closes #1927

This pull request seeks to enhance the i18n strings extraction parser to support string concatenation (BinaryExpression type).

Testing instructions:

Verify that concatenated strings (like that of the InvalidBlockWarning component) are included in the generated languages/gutenberg.pot file when running npm run gettext-strings.

[codecov]

Codecov Report

Merging #2655 into master will increase coverage by 0.06%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #2655      +/-   ##
==========================================
+ Coverage   31.38%   31.45%   +0.06%     
==========================================
  Files         177      177              
  Lines        5413     5418       +5     
  Branches      949      950       +1     
==========================================
+ Hits         1699     1704       +5     
  Misses       3139     3139              
  Partials      575      575

Impacted Files	Coverage Δ
i18n/babel-plugin.js	39.58% <85.71%> (+3.31%)	⬆️

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[BE-Webdesign]

Verify that concatenated strings (like that of the InvalidBlockWarning component) are included in the generated languages/gutenberg.pot file when running npm run gettext-strings.

Verified that this works.

[BE-Webdesign]

It is however copying over the first line break as well:

#: editor/modes/visual-editor/invalid-block-warning.js:32
msgid ""
"This block appears to have been modified externally. Overwrite the external "
"changes or Convert to %s to keep your changes."
msgstr ""

Everything is working good though as far as I can tell.

[aduth]

@BE-Webdesign Thanks for the review! The behavior there can be attributed to the gettext parser I believe, specifically its foldLength option:

> var foldLine = require( 'gettext-parser/lib/shared' ).foldLine;
undefined
> foldLine( 'This block appears to have been modified externally. Overwrite the external changes or Convert to %s to keep your changes.', 76 );
[ 'This block appears to have been modified externally. Overwrite the external ',
  'changes or Convert to %s to keep your changes.' ]

[aduth]

Just in case, I manually debugged the running parsing code and confirmed there are no newlines in the original extracted text argument.