Add support for config.isStaging

[d30jeff]

Motivation

We have a use case where we have a check to enable rate limiter in non-development environment,
and at the same time disable the Redoc Open API documentation in staging environment.

TL;DR: We have a use case where we want to be able to explicitly check for staging environment.

Changes

const config = cleanEnv({ NODE_ENV: 'staging' }, {})

config.isStage // true
config.isStaging // true

Similar issues:

• Why limiting NODE_ENV to development,test,production ? #73