Add JetBrains support

JetBrains has been providing us with free licenses for many years. They have asked us this:

> One small ask before I extend the subscriptions: would you be open to adding a “Supported by JetBrains” mention to your repo or on the website? It helps make the collaboration visible and supports the program. Here’s the snippet most participants use:
>
> ### Supported by
> [](https://jb.gg/OpenSource](https://resources.jetbrains.com/storage/products/company/brand/logos/jetbrains.svg)%5D(https://jb.gg/OpenSource))
> Totally optional — we’ll be happy to extend your subscriptions either way. Looking forward to hearing from you!

I don't have a strong opinion either way.