Setup check to identify unrecognized `config.php` entries

While unrecognized config/config.php entries are ignored, they still create problems and detecting their presence can be a helpful troubleshooting technique.

Their presence can indicate:

• administrative mistakes (i.e. misunderstandings or even just simple typos)
• looming future problems not otherwise identifiable (i.e. recognized config parameters can get incorrect values assigned to them as a result of mistakes in unrecognized entries)

They can mislead:

• seemingly reasonable looking albeit not-at-all-actually-doing-anything configuration entries

They can break things:

• the right typo or cut-n-paste mistake can disable or split an existing parameter's values
• they can overlap with potentially future added parameters

There are some blockers and "gotchas" for doing this, so just noting this as an idea for the future.

Start by tracing how config/config.php is loaded and locating any existing setup checks or configuration validation tests. Review the blockers and gotchas before defining the approach. Done would mean an agreed, tested way for setup to identify unrecognized entries without breaking valid configuration.