Some more variables
Add some more read-only variables into the updater's language:
- self_version: The version of the package where the updater itself lives (compiled into it somehow).
- config_version: The version of config language.
- features: set of important features supported by the updater. Each item is a string ‒ name of the feature.
The goal is to be able to recognize and avoid certain parts of the config files if that would prevent the updater from working.