arbitrary file descriptor source and named descriptors

There is only support for redirecting from `stdout` and `stderr`. We need to support any file descriptor number (up to 10 I think?) as well as named descriptors.

Missing support for syntax:

```sh
command 5> file
command {fd_name}> file
```

See also: https://www.gnu.org/software/bash/manual/html_node/Redirections.html

Start by tracing the shell's existing stdout and stderr redirection handling, then compare it with the Bash redirection rules linked in the issue. Exercise the implementation with `command 5> file` and `command {fd_name}> file`; done means arbitrary file descriptor numbers and named descriptors work alongside stdout and stderr.

Written by the indexing model from the issue text.