Add support for Jetbrains Junie

[wmeints]

As the folks at Jetbrains are ramping up with their new assistant called Junie, I want to take a look at adding support for it here.
Initially, we need to make sure people understand that Junie has a different support model. The folks at Jetbrains test the assistant with specific languages to get the best results possible. We need to mention this in the README.

Required features

• Junie needs to support custom prompts. As of today, this isn't the case. It makes it a lot harder to work with the context engineering prompts.
• Junie needs support for custom instructions. This is documented here: https://www.jetbrains.com/help/junie/customize-guidelines.html#-1bitk0_5

As soon as Junie supports custom prompts we can add support for it here.