Add actions for plugins to register frontend and editor assets

[nylen]

Adds actions so that someone can hook into where Gutenberg styles and scripts are loaded and bring in styles and scripts of their own, for both the frontend and the editor.

This modifies functionality added in #1418, adding a Gutenberg-specific action intended for plugin use in enqueuing front-end assets. It also adds a corresponding action for plugins to enqueue editor assets.

See also #422, #514, and #1420.

[nylen]

As of 00e7c3a there are two actions here:

• enqueue_block_assets - fires in both editor and admin contexts to allow enqueuing assets common to both. If enqueuing scripts here, plugin authors should use an if ( ! is_admin() ) check to ensure that they are only loaded on the frontend. We may want a third enqueue_block_frontend_assets action instead, but this should be added later if necessary.
• enqueue_block_editor_assets - fires in the editor context to allow enqueuing scripts and styles specific to the editor (register the block, build and style its editing controls, etc.)

[swissspidy]

We may want a third enqueue_block_frontend_assets action instead, but this should be added later if necessary.

Then gutenberg_frontend_scripts_and_styles is not an ideal function name when it's called in the admin as well.

[nylen]

Then gutenberg_frontend_scripts_and_styles is not an ideal function name when it's called in the admin as well.

👍 Changed to common in 214e177 [and further improved in def57fc].

[youknowriad]

LGTM 👍

[swissspidy]

This should be a multi-line comment, e.g.

/*
 * ddd
 */

(with one asterisk on the first line)