Add pipefail module

[132ikl]

Allows users to trade external command streaming for pipefail. This is a workaround until we have proper pipefail handling.

Still need to add docs/tests.

Usage:

use std-rfc/pipefail *
^false | print foo; print bar
# => Error: 
# =>   × External command failed
# =>    ╭─[entry #3:2:2]
# =>  2 │ ^false | print foo; print bar
# =>    ·  ──┬──
# =>    ·    ╰── command had a non-zero exit code
# =>    ╰────

[WindSoilder]

I think it's still different to bash's pipefail.
In bash, when running false | echo aa, aa will be printed.

[132ikl]

Oh, that's a good point. I tried to think of a way that a "collecting" version of pipefail could be compatible with a streaming version of pipefail (what bash does), but I think they're actually fundamentally different. I think that if we introduced some interim pipefail mechanism it needs to be compatible with whatever future (streaming-compatible) pipefail implementation we have, so I don't think this is going to work.