Permit configuration via a stand-alone TOML file (`mypy.toml`)

Feature

In #5205, MyPy gained support for declaring configuration in the TOML format, but only in pyproject.toml. I propose supporting configuration via a stand-alone TOML document, such as mypy.toml or .mypy.toml.

Pitch

This parallels, and perhaps would eventually replace, configuration via mypy.ini. This is most useful when checking non-project code, such as in Python (https://github.com/python/cpython/pull/129708), where we have several distinct mypy.ini files to support type-checking different directories of a larger project. It may be useful for mono-repo layouts, where again the use of pyproject.toml is undesirable. Finally, this would be useful to keep tool configuration to independent files — in the Sphinx project, there are several hundred lines of mypy configuration in pyproject.toml, dominating the content of the file.

A

Start by reading the existing TOML configuration support introduced in #5205 and trace how configuration files are discovered and parsed. Add support for a standalone mypy.toml or .mypy.toml while preserving the existing pyproject.toml behavior; done means the standalone file is recognized and its settings are applied.