Include both shell and ps1 entrypoint scripts with `gleam export erlang-shipment`

[Ummon]

On Windows when the command gleam export erlang-shipment is used a PowerShell script is generated as /build/erlang-shipment/entrypoint.ps1. It would be interesting to be able to choose the type of script, for example if you develop on Windows and deploy to Linux.

I propose to add a parameter to set the script type:

 gleam export erlang-shipment --entry-point-language <SCRIPT_LANG>

where <SCRIPT_LANG> can be "powershell" or "bash".

The default behavior (without the parameter) wouldn't be modified.

I can make the changes myself and propose a PR.

[lpil]

Let's always render both!