Add the block registration RFC (#13693)

* Add the block registration RFC

* Update docs/rfc/block-registration.md

Co-Authored-By: youknowriad <benguella@gmail.com>

* Fix typo in WordPress name

* Clarify some points in requirements

* Update the note about REST API

* Removed comments from json based examples

* Fix icon examples

* Add missing comma in JSON example

* A few smaller tweaks

* Fix to deprecated versions example

* Update block-registration.md

* Update block-registration.md

* Update block-registration.md

* Update block-registration.md

* Update block-registration.md

* Apply suggestions from code review

Co-Authored-By: gziolo <grzegorz@gziolo.pl>

* Update docs/rfc/block-registration.md

Co-Authored-By: gziolo <grzegorz@gziolo.pl>

* Update block-registration.md

* Add missing `parent` attribute

* Update block-registration.md

* Add backward compatibility section

* Apply suggestions from code review

* Align the name of style variations with the existing usage

* Update RFC with all details about handling scripts and styles

* Apply suggestions from code review

Co-Authored-By: Jon Surrell <jon.surrell@automattic.com>
Co-Authored-By: Andrew Duthie <andrew@andrewduthie.com>

* Replace spaces with tabs for identation

* Update docs/rfc/block-registration.md

Co-Authored-By: Andrew Duthie <andrew@andrewduthie.com>

* Remove debugging code from test plugin

* Try simplify the way WPDefinedAsset is defined

* Docs: Remove `render_callback` property from the initial version of RFC

* Further simplify the definition of assets and clarify how it works with WordPress

* Apply suggestions from code review

Co-Authored-By: Jorge Bernal <jbernal@gmail.com>
Co-Authored-By: Andrew Duthie <andrew@andrewduthie.com>

* Remove the section about PHP runtime