Support ${VAR_NAME} syntax in any config string value, substituted at
config load time. Useful for referencing API keys and secrets from
environment variables without hardcoding them in the config file.
- Only uppercase env vars matched: [A-Z_][A-Z0-9_]*
- Missing/empty env vars throw MissingEnvVarError with path context
- Escape with $${VAR} to output literal ${VAR}
- Works with $include (included files also get substitution)
Closes#1009
