Specify a directory.

Takes an optional subpath, then one or more callbacks.

Returns function which creates directory at specified subpath, then pipes it through the callbacks.

Example: const makeEmptyDir = Dir('foo'); await makeEmptyDir(); // created "./foo/"

const makeDirWithFiles = Dir('foo', Txt('hello.txt', 'world'));
await makeDirWithFiles({ cwd: '/path/to/somewhere/' });
// created "/path/to/somewhere/foo/hello.txt" and wrote "world'