src/vs/base/common/extpath.ts/isRootOrDriveLetter
export function isRootOrDriveLetter(path: string): boolean {
	const pathNormalized = normalize(path);

	if (isWindows) {
		if (path.length > 3) {
			return false;
		}

		return hasDriveLetter(pathNormalized) &&
			(path.length === 2 || pathNormalized.charCodeAt(2) === CharCode.Backslash);
	}

	return pathNormalized === posix.sep;
}