coreutils' `--reflink` behavior with `cp`

Related problem

On some filesystems that support Copy-on-Write (CoW), GNU coreutils' cp supports "instant" copy by default, but this can be optionally disabled by supplying --reflink=never to create an actual duplicate block.

Based on some simple testing, it seems like nu's cp does CoW copy when -p is not supplied, but does actual copy when -p is supplied. However, it doesn't seem like this is documented in the help or the docs.

Describe the solution you'd like

Make an option, cp --nocow, that allows users to manually specify the behavior of cp on CoW systems, similar to coreutils' one.

Describe alternatives you've considered

Alternative would be to just document the -p's behavior to not use CoW, but this is unideal:

• -p option should only do what the option suggests it will do, which is showing the progress, with no side effects.
• When a user is copying a large directory, they might want to observe the progress, but would want the operation to finish ASAP, utilizing CoW whenever possible.

Additional context and details

I have tested this on Btrfs, using following commands:

# create a random file
dd if=/dev/urandom of=~/Desktop/testfile bs=1M count=16384
# copy w/o `-p` option
cp testfile testfile_cow
# copy w/ `-p` option
cp -p testfile testfile_nocow
# observe the `Exclusive` column of output
btrfs filesystem du .

Start with Nushell's cp command implementation and its help output, then read the cp documentation at nushell.sh/commands/docs/cp.html. Reproduce the Btrfs behavior from the issue and determine where copy mode is selected. Done means cp exposes a documented --nocow option with behavior that can be verified against the CoW and non-CoW examples.