This one script can be saved as any of [.cmd, .bat, .ps1, .sh] and will run under cmd.exe, powershell, pwsh, bash and zsh.

test.ps1

#!/usr/bin/env -S bash # > NUL 2>&1 || echo off && goto init:
function shh() { return; } ; shh \\<<shh
## THIS IS THE START OF THE POWERSHELL SCRIPT #################################
write-host "Setting variable in powershell"
$env:abc=300

return;
## THIS IS THE END OF THE POWERSHELL SCRIPT ###################################

<#
shh
## THIS IS THE START OF THE POSIX SCRIPT ######################################

# check to see if we've been dot-sourced
sourced=0
if [ -n "$ZSH_EVAL_CONTEXT" ]; then
  case $ZSH_EVAL_CONTEXT in *:file) sourced=1;; esac
elif [ -n "$BASH_VERSION" ]; then
  (return 0 2>/dev/null) && sourced=1
else # All other shells: examine $0 for known shell binary filenames
  # Detects `sh` and `dash`; add additional shell filenames as needed.
  case ${0##*/} in sh|dash) sourced=1;; esac
fi

if [ $sourced -eq 0 ]; then
  echo 'This script is expected to be dot-sourced'
  echo ''
  echo "You should instead run '. $(basename $0)'"
  exit
fi

echo "Setting variable in posix"
abc=300

return 
## THIS IS THE END OF THE POSIX SCRIPT ########################################


## THIS IS THE START OF THE CMD SCRIPT ########################################
:init 

echo "Setting variable in CMD"
set abc=300

: THIS IS THE END OF THE CMD SCRIPT ###########################################
:#>