Config validation: enforce top-level options

[rarkins]

Describe the proposed change(s).

Config options like baseBranches should only be configured at the root/top-level of a config, but we don't have an automated way to document and validate this.

We already have a parents field in config options which is currently used to tie options to non-root objects like packageRules.

I propose:

• Designate a new term to mean "root". Could be root, top-level, or .
• Whenever an object has parents=root, raise a config warning error if it's found elsewhere, e.g. in packageRules

Probably done later:

• Enforce the same in JSON schema