Allow a user to specify the npm script that runs in the launch.toml

[lholmquist]

Using the current Node.js buildpack, there doesn't seem to be a way to change the npm script that is run from the launch.toml. It is hard coded to use npm start. https://github.com/boson-project/buildpacks/blob/main/buildpacks/nodejs/bin/build#L34

This is fine for most things, but i wanted to start the node process with the --inspect flag so i could connect to it with the Chrome Debugger.

There should be someway to pass in some type of Environment variable that could potentially change this.

@lance ^^