Deprecate configurable.environment

[Mortaro]

Nullstack has a redundant feature that allows you to define secrets and settings by environment like:
settings.production.name = "Tinky Winky"

This was useful before Nullstack had proper environment support and could be deprecated now so the user can set secrets like:

secrets.name = environment.production ? "Tinky Winky" : "Dipsy"

Also the user will probably just use .env instead

Hints 1:
https://github.com/nullstack/nullstack/blob/master/server/configurable.js
All this could be deleted and replaced with simple objects. Freezing could become a simple Object.freeze.

Tests:
https://github.com/nullstack/nullstack/blob/master/tests/src/ContextSecrets.test.js

I want to remove unneeded features before i claim 1.0