dotenv: Warn about using DENO_ prefixed variables

[ycmjason]

I recently encountered an issue while trying to use the DENO_KV_PATH environment variable that will be used in openKv on Deno Deploy.

The problem is that it consistently fails, and after checking the logs, it appears that Deno Deploy doesn't allow environment variables prefixed with DENO_. The error message I received was:

Env vars cannot begin with DENO_

Upon renaming the env variable to CUSTOM_DENO_KV_PATH it starts working again.

I haven’t seen this limitation documented anywhere for Deno Deploy, but I think it’s something worth highlighting. It would also be helpful if dotenv could issue a warning when this happens.

Just thought it might be useful to bring this to your attention.