Move this feature from a module preset to a regular option, so that it's
easily discoverable and accessible.
Simplify the implementation of `generateSecrets` by adding it to the
existing `setup-secrets` service script.
Also rename option setup-secrets -> setupSecrets.
Previously, the sessions contained only explicitly exported variables
and functions.
This was fragile and in part buggy due to lacking exports.
Interactive features like user-defined aliases and functions are still
working as before.
