nvm.sh uses `NVM_SCRIPT_SOURCE="$_"` to detect its source location.
Adding `: nvm.sh` before each source line ensures `$_` is set correctly, preventing breakage when the previous command (e.g., `set -ex`) overwrites it.
When sourcing a script without parameters in zsh the sourced scripts
gets the same parameters as the sourcing file and is able to modify
these parameters. Prevent nvm from removing all parameters of sourcing
script by processing a copy of the parameters in a function.