Paste caption shortcode

[ellatrix]

This is a first attempt at handling pasted shortcodes.

• Shortcode handling needs to occur before anything else, otherwise we risk losing important info or splitting the shortcode.
• I've created block transforms for this, so it will the the image block that handles the caption shortcode, gallery that handles the gallery shortcode etc.
• I'd like to regard the shortcode attributes as the "sourced" attributes we have for blocks, then parse the content of the shortcode with hpq.

While I used the core shortcodes script in this PR, I think we should bundle it so that we can write tests.

[codecov]

Codecov Report

Merging #2478 into master will increase coverage by 0.22%.
The diff coverage is 73.33%.

@@            Coverage Diff            @@
##           master   #2478      +/-   ##
=========================================
+ Coverage   26.88%   27.1%   +0.22%     
=========================================
  Files         158     160       +2     
  Lines        4892    5109     +217     
  Branches      814     872      +58     
=========================================
+ Hits         1315    1385      +70     
- Misses       3034    3143     +109     
- Partials      543     581      +38

Impacted Files	Coverage Δ
blocks/editable/index.js	11.06% <0%> (-0.05%)	⬇️
blocks/api/paste/strip-wrappers.js	100% <100%> (ø)	⬆️
blocks/api/paste/normalise-blocks.js	96.87% <100%> (+0.2%)	⬆️
blocks/api/paste/strip-attributes.js	100% <100%> (ø)	⬆️
blocks/api/paste/convert-shortcodes.js	44.44% <44.44%> (ø)
blocks/library/image/index.js	14.63% <50%> (-0.37%)	⬇️
blocks/api/paste/index.js	95.23% <95.23%> (-4.77%)	⬇️
editor/header/saved-state/index.js	75% <0%> (-10.72%)	⬇️
blocks/library/heading/index.js	18.91% <0%> (-4.9%)	⬇️
editor/modes/visual-editor/block.js	0% <0%> (ø)	⬆️
... and 8 more

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[ellatrix]

Closing in favour of #2874.