Allow dataclasses in settings

[FriedrichFroebel]

This allows using dataclasses.dataclass inside the settings to allow clean access for more complex data structures (in my case: a list of images configured through an external file which are iterated over inside the theme). Previously, this would fail with a strange AttributeError. Some more background on this is available in mkdocs/mkdocs#3141 for example

Pull Request Checklist

• Ensured tests pass and (if applicable) updated functional test output
• Conformed to code style guidelines by running appropriate linting tools
• Added tests for changed code
• Updated documentation for changed code

[justinmayer]

Thank you for the enhancement, Friedrich.