sigil to indicate reading contents from a process

options like --data are moving toward requiring a sigil to indicate that their contents need to be pulled from a file. We could expand this to a new sigil to indicate that the argument is a program to execute, the output of which should be used as the real content. This idea came from a user thinking that's what --pipe did. This is mostly unnecessary as backticks would usually do the same thing, but it would be nice to have the functionality on windows

Example:
```
--body "some contents"
uses "some contents"

--body @file.txt
uses contents of file.txt

--body :"tail -1 file.txt"
uses the output of "tail -1 file.txt"

Start by locating the command-line option parsing and the existing --data handling for @file arguments. Define how the proposed :"command" form should execute and capture output, including Windows behavior, then verify that literal values and file-based values remain unchanged.

Written by the indexing model from the issue text.