Closes #4770

(really closes issue described in [this
comment](https://github.com/zed-industries/zed/issues/4770#issuecomment-2258728884)
on #4770)

Only implemented for MacOS and Linux for now as I have no way to test on
Windows or BSD.
PRs welcome!

Release Notes:

- Added support for reading from anonymous file descriptors (e.g.
created as part of process substitution) on MacOS and Linux