Components: Extract TableOfContents as reusable component

[youknowriad]

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

Testing instructions

• Check that the "info" button style works (header toolbar)

[youknowriad]

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

[codecov]

Codecov Report

Merging #3639 into master will increase coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #3639      +/-   ##
==========================================
+ Coverage    36.8%   36.85%   +0.04%     
==========================================
  Files         268      268              
  Lines        6649     6640       -9     
  Branches     1201     1201              
==========================================
  Hits         2447     2447              
+ Misses       3550     3541       -9     
  Partials      652      652

Impacted Files	Coverage Δ
editor/components/table-of-contents/index.js	0% <ø> (ø)
editor/edit-post/header/header-toolbar/index.js	0% <0%> (ø)	⬆️

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