build/lib/extensions.ts/isWebExtension
function isWebExtension(manifest: IExtensionManifest): boolean {
	if (typeof manifest.extensionKind !== 'undefined') {
		const extensionKind = Array.isArray(manifest.extensionKind) ? manifest.extensionKind : [manifest.extensionKind];
		return (extensionKind.indexOf('web') >= 0);
	}
	return (!Boolean(manifest.main) || Boolean(manifest.browser));
}