Add check that active configuration directory is writable to system_requirements

Description of the need

I've gotten bitten a few times when file ownership or permissions gets set incorrectly on config/active such that the active configuration directory isn't writable. Since some modules still incorrectly store states in config instead of in the states storage system, that can lead to surprisingly broken sites with entire pages failing to load and displaying "Failed to write configuration file" errors.

Proposed solution

I suggest adding a check to make sure that the active configuration directory is writable to the system requirements function. That would display an alert on the Backdrop status page and could easily be detected by monitoring systems.

Alternatives that have been considered

I'm going to work on adding this check to our sites via a custom module, but this seems like an issue universal enough to merit addition to core.