[FR]: support flattened toml project configuration

Run `cargo sane check` results an error with `Cargo.toml` file which has flattened configs:

```
❯ cargo sane check

🧠 cargo-sane check

Error: Failed to parse Cargo.toml

Caused by:
    TOML parse error at line 4, column 1
      |
    4 | version.workspace = true
      | ^^^^^^^
    invalid type: map, expected a string
```
