feat: add support for Billie

[LinusU]

Summary

I've added support for the Billie payment method.

Motivation

I need to use Billie in our React Native app.

Testing

• [x] I tested this manually
• [ ] I added automated tests

Documentation

Select one:

• [ ] I have added relevant documentation for my changes.
• [ ] This PR does not result in any developer-facing changes.
• [X] I'm not sure where to add documentation

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Please let me know if there is anything more I can do! E.g. I'm not sure where to add documentation, I searched the repo for "klarna" and made sure to implement Billie in all the same places, but there was no docs...

I also added the code after Bancontact, since that sorts lexicographically, but it seemed like the current code isn't sorted by anything special. Happy to update if you want it in any other way...

Fixes #1953