Detect relative URLs in known block attributes

[adamziel]

Gives BlockMarkupUrlProcessor knowledge about which block attributes are designed to contain a relative URL.

Known URL attributes can be assumed to hold a URL and be parsed with the base URL. For example, a /about-us value in a wp:navigation-link block’s url attribute is a relative URL to the /about-us page.

Other attributes may or may not contain URLs, and we cannot assume they do. A value like /about-us could be a relative URL or a class name. In those cases we’ll ignore relative URLs and only detect absolute URLs to avoid treating every string as a URL; this requires parsing without a base URL.

Related to WordPress/wordpress-playground#1780

Implementation details

This PR ships a list of all block attributes that are meant to hold a URL. It's similar to how the HTML spec declares a list of all HTML attributes meant to hold a URL.

It also ships a filter the extenders can use to nudge the URL rewriter to treat a block attribute as either:

$is_relative_url_block_attribute = apply_filters(
	'url_processor_is_relative_url_block_attribute',
	$is_relative_url_block_attribute,
	array(
		'block_name' => $this->get_block_name(),
		'attribute_key' => $this->get_block_attribute_key(),
	)
)

A hypothetical plugin shipping a wp:custom-image block could nudge the URL parser to treat its src attribute as a relative URL by registering the following filter:

<?php
function recognize_custom_image_block_attribute_as_relative_url ( $is_known, $context ) {
	if (
		'wp:custom-image' === $context['block_name'] &&
	     'src' === $context['attribute_key']
	) {
		return true;
	}

	return $is_known;
}

add_filter(
	'url_processor_is_relative_url_block_attribute',
	'recognize_custom_image_block_attribute_as_relative_url',
	10,
	2
);

Other changes

This PR renames two constants for clarity:

• URL_ATTRIBUTES -> HTML_ATTRIBUTES_TO_ACCEPT_RELATIVE_URLS_FROM
• URL_ATTRIBUTES_WITH_SUBSYNTAX -> HTML_ATTRIBUTES_WITH_SUBSYNTAX_TO_ACCEPT_RELATIVE_URLS_FROM
• URL_CONTAINING_TAGS_WITH_SUBSYNTAX -> HTML_TAGS_WITH_SUBSYNTAX_TO_ACCEPT_RELATIVE_URLS_FROM

Testing

CI – see the new tests shipped with this PR.