[1.x] Better Environments

[armiol]

This changeset is a transfer of #733 to v1 branch.

Please note, this is a breaking API change.

In particular:

• environment-related types were moved under io.spine.environment package;
• Production was renamed to DefaultMode;
• it is now possible to check whether the environment is enabled; see EnvironmentType.enabled();
• it is also possible to configure a callback executed upon the environment type detection; see EnvironmentType.onDetected(...);
• the API for the custom environment types was changed; see io.spine.environment.CustomEnvironmentType and its documentation for more detail.

Addresses #752.