feat(server): Add dev mode env var

[wtchen]

I'm hoping this small feature will be useful for people to develop against dev and production environments.

This will also help kick off #1321, which needs this distinction.

[bartlomieju]

I believe I was wrong in my original comment in #1321 - Fresh already decides if it is in devmode or not based on DENO_DEPLOYMENT_ID env var.

[wtchen]

Ah, good catch. I think having something explicit would still be useful; I can turn this into an abstraction.

[wtchen]

Ok, I made some changes, let me know what you think 🙂

[marvinhagemeister]

I'm not sure I understand what we gain from exposing if Fresh is in dev mode via an environment variable. Can you share more about your use case?

[wtchen]

Of course. The idea is if the user wants to have different code for dev vs not dev, they can easily check against that env variable.

if (Deno.env.get("FRSH_DEVMODE") === 'true') {
  return new Response('Welcome to my dev site');
} else {
  return new Response('Welcome to my production site');
}

[lucacasonato]

Could that usecase be resolved by a per-project env var?