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

[KotlinIsland]

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

[tool.mypy]
sus_sus = true

[tool.basedmypy]
amongus = true

[mypy]
sus_sus = true

[basedmypy]
amongus = true

[DetachHead]

another reason for this is that vscode complains about basedmypy options because they're not in the json schema hosted on schemastore: https://json.schemastore.org/partial-mypy.json