Investigate adding unit testing

[Ernest314]

See:

• https://docs.microsoft.com/en-us/dotnet/core/testing/unit-testing-best-practices
• https://learn.microsoft.com/en-us/dotnet/core/testing/unit-testing-with-dotnet-test
• https://xunit.net/docs/getting-started/netcore/visual-studio

This would be nice to have. The implementation is obvious for things like RecurringEvent but less so for all of the more interactive components (a lot of mocking might be required).

[Ernest314]

xUnit seems like the best framework to use :)

[Ernest314]

Use deepl-mock to mock DeepL API for /translate (otherwise tests will use up the actual character quota). The supported test text is "proton beam".