Sapper dev should support different NODE_ENV (other than development)

[alexbjorlig]

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Is your feature request related to a problem? Please describe. If I wan't to run our Sapper project locally to debug a production issue, I can't use sapper dev because it hard-codes the NODE_ENV to development. I have to use something like export NODE_ENV=production && sapper build && node __sapper__/build.

Describe the solution you'd like Some way to run sapper dev, respecting the NODE_ENV. Maybe just export NODE_ENV=production && sapper dev

How important is this feature to you? This is a major painpoint, because when our team is debugging production related issues, we then constantly have to re-run the sapper build command

Additional context We use the NODE_ENV property to load corresponding secrets, as well as modify the build process of roll-up.