Awareness of Developer Mode within Node-RED

### Description

_From https://discourse.nodered.org/t/feature-request-self-awareness-of-developer-mode/93147_

I'd like my flows to be self-aware of whether it's in Developer Mode or not, specifically so the tests we run can log whether this is the case.

Is there a way to determine this? I'd like to think I could pull a boolean of this from an environmental variable or something, but I can't find anything on this online.

This comes from me wanting to have better awareness of which devices are running in developer mode, as accessed by our production database, not the flowfuse UI. In each 'test' that our NR flows perform, it would be nice to be able to have a flag for whether it is in developer mode or not. We could then do alerting on this and analytics on how many of our tests were done in developer mode and over what time frame, etc. The flowfuse UI only gives a current snapshot, not historical.

This could also allow me to make sure certain flows stop at a certain point if in developer mode, further guarding from running production processes in developer mode accidentally

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@knolleary says: An Env Var would be handy, but this would require restarting NR each time it comes in/out of developer mode - something we don't do today, and would potentially be disruptive to existing workflows.

### Requested By

- https://app-eu1.hubspot.com/contacts/26586079/record/0-1/2454