Add support for Polar.sh

[mikker]

Hey!

Just checking in to report that I've begun the task of adding support for Polar.sh.

They're fairly new on the scene but the team is awesome and very helpful and they look like a very capable competitor, especially by being Merchant of Record, supporting both subscriptions and one-time payments, and having all sorts of integrations that're as easy as click-to-enable.

I'll make a proper PR when I'm nearing anything presentable but until then, wish me luck 😅

[excid3]

Polar looks cool!

I don't think I'll have time to officially maintain, but how making it as a plugin and we can link to it as a community maintained integration?