Fix default environment name

[dvic]

Because Mix.env() is by default :prod for dependencies, locally, the default Sentry environment is :prod, which is not intuitive when you run an app locally in dev mode.

I think the intended behavior was to use the Mix env of the parent app but that's not what's happening right now (not sure if it's possible to get it).

Edit from Andrea (maintainer)

We should require :environment_name to be set, I agree. Let's do a few things.

Tasks

Beta Give feedback

1. Support SENTRY_ENVIRONMENT_NAME and fall back to SENTRY_ENVIRONMENT as environment variables, to comply with other Sentry SDKs. This is not a breaking change.
2. Require :environment_name to be set, and raise if it isn't. This is a breaking change.

[isaacsanders]

This is also strange, because this library uses SENTRY_ENVIRONMENT_NAME, not SENTRY_ENVIRONMENT like all other libraries do.

[yordis]

Kind of related to #503

The whole configuration managements needs some love

[dvic]

🎉