Editor: Backport Elements API updates.

This commit backports the original PRs from Gutenberg repository:

* [WordPress/gutenberg#40260 #40260 Add support for button elements to theme.json]
* [WordPress/gutenberg#40889 #40889 Theme Json: Don't output double selectors for elements inside blocks]
* [WordPress/gutenberg#41140 #41140 Global Styles: Add support for caption elements]
* [WordPress/gutenberg#41160 #41160 Global Styles: Load block CSS conditionally]
* [WordPress/gutenberg#41240 #41240 Global Styles: Button Element: update button element selector]
* [WordPress/gutenberg#41335 #41335 Duotone: Fix CSS Selectors rendered by theme.json duotone/filter settings for blocks on public pages]
* [WordPress/gutenberg#41446 #41446 Block styles: Account for style block nodes that have no name]
* [WordPress/gutenberg#41696 #41696 Global Styles: Allow references to values in other locations in the tree]
* [WordPress/gutenberg#41753 #41753 Elements: Add an API make it easier to get class names]
* [WordPress/gutenberg#41786 #41786 Support pseudo selectors on elements in theme json]
* [WordPress/gutenberg#41822 #41822 Elements: Button - Fix element selectors]
* [WordPress/gutenberg#41981 #41981 Global Styles: Add support for heading elements]
* [WordPress/gutenberg#42072 #42072 Fix link element hover bleeding into button element default styles]
* [WordPress/gutenberg#42096 #42096 Add visited to link element allowed pseudo selector list]
* [WordPress/gutenberg#42669 #42669 Link elements: Add a :where selector to the :not to lower specificity]
* [WordPress/gutenberg#42776 #42776 Theme JSON: Add a static $blocks_metadata data definition to the Gutenberg instance of WP_Theme_JSON]
* [WordPress/gutenberg#43088 #43088 Pseudo elements supports on button elements]
* [WordPress/gutenberg#43167 #43167 Theme_JSON: Use existing append_to_selector for pseudo elements]
* [WordPress/gutenberg#43988 #43988 Styles API: Fixed selectors for nested elements]

Props onemaggie, bernhard-reiter, cbravobernal, mmaattiiaass, scruffian, andraganescu, dpcalhoun, get_dave, Mamaduka, SergeyBiryukov.
See #56467.

git-svn-id: https://develop.svn.wordpress.org/trunk@54118 602fd350-edb4-49c9-b593-d223f7449a82