Components: Extract a reusable MultiBlocksSwitcher component

[youknowriad]

Extract a reusable MultiBlocksSwitcher component to the editor/components folder.
Needed to be able to separate the edit-post and editor module

Expect some similar PRs today, I'm going to merge them as soon as the tests pass, they consist of moving some files around.

Testing instructions

• Select several paragraphs and transform them to a list

[codecov]

Codecov Report

Merging #3641 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #3641      +/-   ##
==========================================
- Coverage   36.83%   36.81%   -0.02%     
==========================================
  Files         270      271       +1     
  Lines        6644     6647       +3     
  Branches     1201     1202       +1     
==========================================
  Hits         2447     2447              
- Misses       3545     3547       +2     
- Partials      652      653       +1

Impacted Files	Coverage Δ
...components/block-switcher/multi-blocks-switcher.js	0% <0%> (ø)
editor/edit-post/header/header-toolbar/index.js	0% <0%> (ø)	⬆️

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