extensions/html-language-features/client/src/customData.ts/collectInExtensions
function collectInExtensions(localExtensionUris: Set<string>, externalUris: Set<string>): void {
	for (const extension of extensions.all) {
		const customData = extension.packageJSON?.contributes?.html?.customData;
		if (Array.isArray(customData)) {
			for (const uriOrPath of customData) {
				if (!isURI(uriOrPath)) {
					// relative path in an extension
					localExtensionUris.add(Uri.joinPath(extension.extensionUri, uriOrPath).toString());
				} else {
					// external uri
					externalUris.add(uriOrPath);
				}

			}
		}
	}
}