A `basedmypy` key in config files for basedmypy exclusive configuration options

Mypy doesn't really care about unrecognized options in the config file, but a feature like this might help with readability.

```toml
[tool.mypy]
sus_sus = true

[tool.basedmypy]
amongus = true
```
```ini
[mypy]
sus_sus = true

[basedmypy]
amongus = true
```

The issue names no files, tests, or entry points. Start by locating configuration parsing and tests for TOML and INI settings, then trace how project-specific options are handled. Done means the shown [tool.basedmypy] and [basedmypy] sections support basedmypy-exclusive options without relying on [tool.mypy] or [mypy].

Written by the indexing model from the issue text.