feat: Add env var placeholders support

[saladware]

This PR adds support for env vars in PicoLimbo config file.

You can now use ${ENV_VAR} inside the config and it will be expanded
before TOML parsing. This is useful for running PicoLimbo in Docker
or other environments where config is driven by env vars.

What’s included:

• ${ENV_VAR} is replaced with the value from the environment
• ${ENV_VAR} can be used to escape and keep it literal
• Errors for missing or invalid placeholders
• No changes for existing configs

Note: I’m still learning Rust, so feel free to point out anything that
could be written in a more idiomatic way 🙂