Compatible with log4rs

There are still many libraries using the `log` facilities to emit debug informations, `tracing-subscriber` could also register a handler to `log` and output together.

This library aims to share the same configuration process like `log4rs`, so I suggest:

1. `trace4rs` could also accept exactly the same configuration format like `log4rs`, which means:
- They should have the same configuration fields
- They should have the same configuration options
- Format should have the same keys
2. `trace4rs` could also handle outputs from `log`, which means:
- `trace4rs` could work just like `log4rs`

The issue names no files, tests, or entry points. Start by locating trace4rs's configuration and output handling, then compare its supported fields, options, format keys, and log integration with log4rs. Done requires compatible configuration behavior and combined log output, but the exact scope needs clarification.

Written by the indexing model from the issue text.