Blocks: Add shortcode block

[aduth]

Closes #1709

This pull request seeks to implement a new Shortcode block:

Testing instructions:

Verify that you can add a shortcode block, and that it is saved into post content with no additional surrounding elements.

1. Navigate to Gutenberg > New Post
2. Insert a Shortcode block
3. Add a shortcode
4. Switch to Text view
5. Confirm the shortcode markup appears as expected (only with block delimiter comments)
6. Press Preview
7. Confirm the shortcode displays as expected

[mtias]

@jasmussen could we get a special icon here?

[bobbingwide]

@mtias @jasmussen Regarding a special icon. See WordPress/dashicons#261.

[jasmussen]

I'll put this on my list, thanks for the reminder!

[codecov]

Codecov Report

Merging #2352 into master will increase coverage by 0.13%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #2352      +/-   ##
==========================================
+ Coverage   24.69%   24.83%   +0.13%     
==========================================
  Files         152      153       +1     
  Lines        4738     4752      +14     
  Branches      799      801       +2     
==========================================
+ Hits         1170     1180      +10     
- Misses       3014     3018       +4     
  Partials      554      554

Impacted Files	Coverage Δ
blocks/library/shortcode/index.js	50% <50%> (ø)
components/higher-order/with-focus-return/index.js	92.85% <0%> (-7.15%)	⬇️

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[mtias]

It seems we have an issue with attaching images to posts because [gallery], without specifying ids, didn't work.

[mtias]

Added a short description.

[lkraav]

Whoa nice, igniting all test procedures.

[aduth]

It seems we have an issue with attaching images to posts because [gallery], without specifying ids, didn't work.

Hmm, this is meant to be working, was part of #1967.