Feature Request: Switch to using sayanarijit/expandvars for as_str_expanded()

Was curious if there was any interest in switching over to using https://github.com/sayanarijit/expandvars as it provides support for standard unix/go env variable expansion (can be helpful in setting default values).

Example of a config file with this syntax:
```
database:
server: ${DATABASE_SERVER:-localhost}
port: ${DATABASE_PORT:-5432}
user: ${DATABASE_USERNAME:-app}
passsword: ${DATABASE_PASSWORD}
```

Start at as_str_expanded() and inspect how configuration values are currently expanded. Compare that behavior with sayanarijit/expandvars, using the issue's DATABASE_* examples as the expected syntax. Done means the proposed expansion behavior is supported and covered by tests for defaults and required variables.

Written by the indexing model from the issue text.