feat(postgres): add `commands` configs

[cdmistman]

right now setup-postgres and start-postgres are added to the user's dev shell, but there's little documentation about even the existence of these utilities. since they're non-standard utilities (that is, they aren't defined by pkgs.postgresql), it's non-ideal leaving them undocumented

this solution isn't ideal because the categories are subjective to my own categorization method, but this greatly improves ux

an even better solution would be to refactor the commands option to accept and attrset of <name> = <command config>, which would allow the user to respec the categories