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

[gziolo]

Now that we are close to be able to expose all block with the new REST API endpoint as proposed in ​https://github.com/WordPress/gutenberg/pull/21065 (#47620), the next step is to make it easier to register blocks on the server.

The initial implementation was proposed in ​https://github.com/WordPress/gutenberg/pull/20794.

It's still being worked on in ​https://github.com/WordPress/gutenberg/pull/22519 where Extend register_block_type_from_metadata to handle assets.

There is also a related task #49615 that proposes register_block_type_args filter and might help improve the general experience when registering a block.

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() );

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

[gziolo]

The patch added on GitHub includes changes added to Gutenberg in:

• ​https://github.com/WordPress/gutenberg/pull/20794
• ​https://github.com/WordPress/gutenberg/pull/22519

register_block_type_from_metadata 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.

@ocean90, is it good to go now?

[gziolo]

In 48141:

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

Backports changes added to Gutenberg in:

• ​https://github.com/WordPress/gutenberg/pull/20794
• ​https://github.com/WordPress/gutenberg/pull/22519

register_block_type_from_metadata function is going to be used to register all blocks on the server using block.json metadata files.

Props ocean90, azaozz, aduth, mcsf, jorgefilipecosta, spacedmonkey, nosolosw, swissspidy and noahtallen.
Fixes #50263.

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

This PR exposes all core blocks on the server to be used with the REST API block types endpoint.

Note: Embeds blocks aren't registered on the server. It's a more complex task for post WordPress 5.5.

## Testing

It can be tested using the temporary approach where blocks are exposed from the server using wp.blocks.unstable__bootstrapServerSideBlockDefinitions function call:


[gziolo]

In 48262:

Editor: Register core blocks on the server

Exposes all core blocks (excluding embeds) on the server to be used with the REST API block types endpoint.

Props spacedmonkey, timothyblynjacobs.
Fixes #50263.

[gziolo]

After working on support for register_block_type_args filter in #49615, I realized that we need to use init action for core blocks to make it possible to use it.

[gziolo]

Move core blocks registration to init hook

[gziolo]

In 48279:

Editor: Move core blocks registration to init hook

After working on support for register_block_type_args filter in #49615, it became clear that we need to use init action for core blocks to make it possible to use this filter.

Fixes #50263.

[justinahinon]

Dev note link: ​https://make.wordpress.org/core/2020/07/30/block-api-updates-in-5-5/