feat: global and local config resolution + add new options: nestingSeparator and transform

[alexmarqs]

Motivation:

• Support nesting delimiter in envAdapter #34
• Support removing prefix in env adapter #35
• Regex is strange #33

With these changes (still a WIP), we’ll be able to:

• Support both adapter-specific and global options. For overlapping options, the adapter-specific ones will take precedence.
• Introduce a new nestingSeparator option, available in the built-in dotEnv and Env adapters, and customizable in any user-defined adapter if needed;
• Add a new transform option, allowing us to filter or modify data before it’s processed. This is useful for tasks like stripping prefixes, etc.

These improvements also lay the groundwork for designing more specialised API options per adapter, using transform as the core mechanism but something for another iteration.

[alexmarqs]

CC: @TimoGlastra @Pfeifenjoy mentioning you as this may relate to your points/suggestions in the issues — any feedback you have would be greatly appreciated (if so, please check the README changes to understand the new capabilities)