Core: Enhance environment variable expansion in configuration

[clemyan]

What's the problem this PR addresses?

Environment variable expansion in config files supports neither nested expansions and robust escaping (e.g. it is impossible to make a config \ followed by the value of an environment variable)

Fixes #6937
Fixes #6998
Supercedes/closes #7005

How did you fix it?

Implement a PDA-like parser to generate the expanded value that:

• Supports nested expansions
  • Unset variables without fallback nested inside unused fallbacks do not throw
• Supports escaping \, $, and }; allowing representation of any desired string/expansion
• Rejects unclosed expansion and unknown expansion syntax (for forward compatibility)

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.

[clemyan]

Oops I botched the docs 😅

[pvcresin]

@arcanis I really need this feature right now, would you be able to check it? 🙇

[clemyan]

@pvcresin You can try it now using yarn set version from sources --branch 7033

[pvcresin]

@clemyan I see. Thank you very much for informing me.

[pvcresin]

@arcanis I apologize for having mentioned you.