Editor: Introduce new API method that register block from block.json metadata file

[gziolo]

Trac ticket: https://core.trac.wordpress.org/ticket/50263

This patch includes changes added to Gutenberg in:

• Block API: Add new utility to register block types from metadata in PHP gutenberg#20794
• Block API: Extend register_block_type_from_metadata to handle assets gutenberg#22519

This is how the interface looks like:

<?php
/**
  * Registers a block type from metadata stored in the `block.json` file.
  *
  * @since 5.5.0
  *
  * @param string $file_or_folder Path to the JSON file with metadata definition for
  *     the block or path to the folder where the `block.json` file is located.
  * @param array  $args {
  *     Optional. Array of block type arguments. Any arguments may be defined, however the
  *     ones described below are supported by default. Default empty array.
  *
  *     @type callable $render_callback Callback used to render blocks of this block type.
  * }
  * @return WP_Block_Type|false The registered block type on success, or false on failure.
  */
function register_block_type_from_metadata( $file_or_folder, $args = array() );

This function is going to be used to register all blocks on the server when backporting the block editor changes in https://core.trac.wordpress.org/ticket/50420.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[ocean90]

DocBlocks needs some minor formatting updates.

[gziolo]

@ocean90, is it good to go now? Thank you for review, by the way 👍

[ocean90]

Two minor changes, looks good otherwise.