Contribution sought: looking for reviewers for a pull request

[machinewrapped]

I have finally got a working branch which restructures the project significantly to make it easier to support multiple translation services:
#125

There is still only one supported TranslationProvider (OpenAI), but settings are now grouped into general and provider-specific sections, which can still come from many sources (defaults, command line, environment variables, saved settings, project settings).

It's a big change so I'd appreciate any eyes I can get on it before merging it back to main.