Add zsh and bash shell completion

[srid]

In the example nix shell, typing , followed by hitting the TAB should auto-complete the user-configured script names (hello and fmt in the case of the example). This should work on both bash and zsh (inasmuch as macOS uses zsh by default).

The code for this could potentially go here:

mission-control/nix/wrapper.nix

Lines 48 to 50 in 3b1c773

	# TODO: bash and zsh completion
	postInstall = (oa.postInstall or "") + ''
	'';

For reference, see how purs-nix implements it:
https://github.com/purs-nix/purs-nix/blob/0fae0fd9316a20e2312cfdbcb8135b0e56330e01/purs-nix-command.nix#L254-L271

[srid]

Should also add -h and --help to the completion list.

[srid]

https://stackoverflow.com/a/74981309/55246

[srid]

We have a larger problem: direnv/direnv#443

Even nix develop doesn't provide shell completion for nativeBuildInputs (eg: exa).

[srid]

The alternative we now recommend supports shell completion through just.

See https://github.com/juspay/just-flake