backticks: implement piping and std{in,out,err} redirection

[StefanKarpinski]

Adds shell-style pipe and redirection operators to backtick command literals. Part of #20401. Examples:

julia> run(`ls -1 | sort -r > /tmp/out.txt`)

julia> read(`cat < /tmp/out.txt | head -5`, String)

julia> run(`make 2> /tmp/errors.txt`)

julia> file = "/usr/share/dict/words"
"/usr/share/dict/words"

shell> tail $file | sort -r
Zyzzogeton
Zyzomys
zythum
Zythia
zythem
Zyryan
Zyrian
Zyrenian
zymurgy
zymotoxic

Supported operators

Syntax	Meaning
cmd1 | cmd2	Pipe stdout of cmd1 to stdin of cmd2
cmd > file	Redirect stdout to file
cmd >> file	Append stdout to file
cmd < file	Redirect file to stdin
cmd 2> file	Redirect stderr to file
cmd 2>> file	Append stderr to file
cmd 1> file	Explicit stdout redirect (same as >)
cmd 0< file	Explicit stdin redirect (same as <)

These lower to pipeline() calls at parse time. Simple commands without operators still return Cmd as before.

Display round-tripping

Pipeline and redirect objects display using backtick syntax when possible:

julia> `echo hello | sort > out.txt`
`echo hello | sort > out.txt`

julia> `make 2> errors.txt`
`make 2> errors.txt`

Commands with non-representable features (e.g. custom env, setuid) fall back to the existing pipeline(...) display format.

REPL shell mode

Shell mode now handles pipes and redirects. Before:

shell> echo hello | wc -c
hello | wc -c

After:

shell> echo hello | wc -c
6

In other words, previously shell mode would pass the | as a literal argument to the echo command; now it constructs a pipeline just as run(`echo hello | wc -c`) would.

Implementation notes

• Operators inside quotes are treated as literal characters ('|', '>'), matching shell behavior
• Fd prefixes require unquoted integers: '2'>file redirects stdout with argument "2", not stderr (POSIX-compliant)
• Filenames in redirects support $ interpolation
• Stderr redirects use a separate pipeline() call so stdout and stderr can have independent append modes

This PR was written with the assistance of generative AI.

[StefanKarpinski]

Converted to draft since it should not be merged until #61397 is (or not).

[StefanKarpinski]

Triage likes being able to use the syntax, but is unsure about the change to printing of pipeline objects, but may come around. The conservative approach here may be to revert the printing change part, merge that, and then open the printing change as its own, separate PR that we can decide later on if we want or not.

[StefanKarpinski]

The argument for printing as explicit pipeline calls (aside from not changing printing being more conservative inherently), was that Julia programmers are expected to know how to construct these things programmatically, so showing that to them seems reasonable. My counterpoint was that I still cannot remember how to construct moderately complex pipelines using Julia syntax, whereas I very much know how to do it with shell syntax.