Breaking: #107927 - Remove "external" property / option from TypoScript and AssetRenderer 

Description 

The resource property external in the PAGE properties includeCSS, includeCSSLibs, includeJS, includeJSFooter, includeJSFooterlibs and includeJSLibs is now obsolete.

This also obsoletes the External option in AssetRenderer.

Both are removed in favor of the new unified URI resource definition.

Instead of marking URIs as URIs with an additional option, prefix the URI, that shall be used with URI:, or simply use absolute URLs starting with http(s)://, where the prefix is not required.

This URI resource definition will work across the system, not only in TypoScript or AssetCollector/ AssetRenderer.

Impact 

Using the external property in TypoScript or the external option in AssetCollector will have no effect any more. If absolute URLs have been used as resources, everything will work as before. Relative URIs must be prefixed with URI: from now on, otherwise an exception is thrown.

Additionally, the string after the URI: keyword must be a valid URI, otherwise an exception is thrown as well. Before this change, invalid URIs (marked as external) would have been rendered to HTML, without any obvious feedback for developers or integrators. Browsers then ignored such invalid references.

Affected installations 

TYPO3 installations using the external property in TypoScript or the external option in AssetCollector.

Migration 

TypoScript before:

page = PAGE
page.includeCSS {
      main = https://example.com/styles/main.css
      main.external = 1
      other = /styles/main.css
      other.external = 1
}

TypoScript after:

page = PAGE
page.includeCSS {
    main = https://example.com/styles/main.css
    other = URI:/styles/main.css
}

PHP Code before:

$assetCollector->addStyleSheet(
    'myCssFile',
    '/styles/main.css',
    [],
    ['external' => true]
);

PHP Code after:

$assetCollector->addStyleSheet(
    'myCssFile',
    'URI:/styles/main.css',
);