Switch pigeons to kotlin and swift

[andreasgangso]

Why? Mostly maintainability.

• It's easier to understand when searching and navigating into, and easier to get the signatures correct etc when overloading, etc.
• Unverified assumption: Effort will probably be put into kotlin/swift from flutter's side, as that's what most all new projects are going to choose when setting up?
• Unverified assumption: It's probably better interop if the generated code is the same as the code we write

Also I've seen the pigeon maintainer asking people why someone would use java if their implementation is in kotlin and such, at least a couple of times, e.g. here.