Gutenberg: set block_width global

[nickcernis]

Gutenberg has an upcoming block_width global that child themes will need to override.

Example:

$GLOBALS['block_width'] = array(
          'default' => 640,
          'wide'    => 800,
          'full'    => 1024,
);

Or the proposal in this comment:

set_block_width(
    array (
        'default' => 600,
        'wide' => 780,
        'full' => 1024,
    )
);

I'm opening this to track the progress of this feature so we can add the relevant function call/global override once it's merged in WP core/Gutenberg.

For reference:

• Enhancement: images handling WordPress/gutenberg#6177
• https://github.com/WordPress/gutenberg/blob/5102f35b412ac6af825f0951cf3c0c2c2f62b5ed/packages/block-library/src/image/index.php#L30