Safer access to settings (validation, typehinting and default values placed in a single place)

Things done:

• separate env for dev settings (so now things like force_onboarding or debug loop are not stored next to other settings in the settings.toml file more important for user)
• introduced SafeSettings class which should be now used for getting settings. compared to previous approach it has some advantages:
  • is fully type-hinted so there will be no typo or receiving the wrong type possible
  • does validation on setting access and initially right after clive launch. Previously it was possible to have an error later in a misleading place which would require harder debugging
  • default values (when no setting is found in the settings.toml file) are now defined in a single place. Previously there were multiple places like settings.get("NODE.REFRESH_RATE", 1.5)
  • Note: in the future (probably after dynaconf 4.0.0, the approach of defining Validators via dyanconf and having official stable typed dynaconf might be better solution)
• replaced all dynamic settings change from settings.SOMETHING = "new" or settings["SOMETHING"] = "new" to settings.set("SOMETHING", "new") as it is better searchable