src/vs/base/common/uri.ts/URI/joinPath
class URI:	static joinPath(uri: URI, ...pathFragment: string[]): URI {
		if (!uri.path) {
			throw new Error(`[UriError]: cannot call joinPath on URI without path`);
		}
		let newPath: string;
		if (isWindows && uri.scheme === 'file') {
			newPath = URI.file(paths.win32.join(uriToFsPath(uri, true), ...pathFragment)).path;
		} else {
			newPath = paths.posix.join(uri.path, ...pathFragment);
		}
		return uri.with({ path: newPath });
	}