Plugin configuration for the mypy plugin and docs

[binaryDiv]

IMPORTANT: This PR is marked as a draft because the branch is based on another branch: mypy-plugin.
It should be rebased on dev-mypy and only merged after that branch has been merged.

---

This is a part of making validataclass mypy-compatible (#116).
Please note that the target branch is dev-mypy instead of main.

---

This PR extends the mypy plugin introduced in #140 (not merged as of now) by a simple config system. It also adds some documentation on how to use the mypy plugin including the newly added settings.

The plugin config currently only supports the pyproject.toml format, which I think is fine because that's the recommended way to configure tooling for Python projects. The plugin automatically detects the config file used by mypy and reads the same file (if it ends on .toml).

The primary purpose of the plugin config is to support custom decorators and other extensions to validataclass, like those needed for the validataclass-search-queries library, or custom decorators that we have in some of our company's projects.

The PR introduces a conditional dependency to the library: For Python <3.11 (meaning 3.10), the library now requires the tomli library to parse TOML files. Since Python 3.11, the standard library contains tomllib which is fully API-compatible to tomli (I think it's literally the same library?). The tomli library itself is a simple library without any dependencies, so it stays lightweight.

[ninanomenon]

Do you want to solve this TODO in this MR or is that something for later?

[binaryDiv]

Something for later. Would love to do it now, but I've already been working way too long on this whole plugin. :/

And the config parsing is tested "good enough" for now, there's the mypy tests that make use of the config and I'm already using it in validataclass-search-queries. I'd just like to add some more unit tests later to fully cover it with unit tests.