added option programs.bash.promptInit

[christian-burger]

Option is identical to NixOS option. Default value copied from there.

Motivation: I know zsh is default in macOS. But every time you enter nix develop, there is only a rudimentary prompt by default. Plus, on older MacBooks some people might not have transitioned to zsh, yet. Both zsh as well as fish provide this option as well.

Only problem is: This might break things for people using programs.bash.interactiveShellInit or environment.interactiveShellInit to configure their prompt, because it gets overwritten as I imitated the way it is implemented in the zsh module. I could, alternatively to the way it is done now,

1. move it up, so that it is executed during shell initialization before said option. Or
2. provide a warning in the module, that alerts the user of the interactiveShellInit option to the change the next time they run darwin-rebuild.

Though the latter is more consistent with expected behaviour in my opinion, it will have to be removed after some grace period.