Add "caption" styles in images and embeds to the front-end.

[mtias]

Make sure these arrive on the front end and are explicit:

[mtias]

@karmatosed a theme would overwrite these with .wp-block-image figcaption {}.

[codecov]

Codecov Report

Merging #2345 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2345      +/-   ##
==========================================
- Coverage   24.69%   24.63%   -0.07%     
==========================================
  Files         152      152              
  Lines        4738     4949     +211     
  Branches      799      847      +48     
==========================================
+ Hits         1170     1219      +49     
- Misses       3014     3132     +118     
- Partials      554      598      +44

Impacted Files	Coverage Δ
blocks/library/embed/index.js	48.36% <ø> (+2.91%)	⬆️
blocks/library/image/index.js	12.04% <ø> (-0.2%)	⬇️
blocks/library/separator/index.js	42.85% <0%> (-7.15%)	⬇️
blocks/library/cover-text/index.js	29.41% <0%> (-5.59%)	⬇️
blocks/library/heading/index.js	18.91% <0%> (-4.9%)	⬇️
blocks/library/latest-posts/index.js	7.22% <0%> (-2.78%)	⬇️
blocks/library/gallery/index.js	23.8% <0%> (-1.2%)	⬇️
blocks/library/list/index.js	6.72% <0%> (-0.51%)	⬇️

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