feat: prefixes for config keys

[Christoph-Harms]

This adds the possibility to load config files and prefix the keys from the loaded file. This is meant to prevent conflicts when multiple loaded config files contain the same keys.

[Christoph-Harms]

Closed, because suboptimal. After a bit more thought has gone into this, a v3 of envi will be made where this potential conflict of var names is prevented without having to virtually double the number of methods. In the meantime, the Mutex will be added to the main branch and released as v2.2.1, while this prefix solution will be released as v2.2.1-prefix.0.