codegen-java/kotlin: Support generating sealed classes

[odenix]

It would be very nice to generate sealed Java/Kotlin classes whenever possible. This would improve code comprehension, pattern matching, and the IDE experience.

Potential solutions:

1. Infer which abstract and open Pkl classes can be sealed in Java/Kotlin.
   At a minimum, this would require passing all Pkl modules at once to Java/KotlinCodeGenerator so that it can check if a class is subclassed in another module. Currently, Java/KotlinCodeGenerator only accepts a single module whereas CliJava/KotlinCodeGenerator accepts multiple.
2. Support sealed classes in Pkl. This feels like a cleaner solution and could prove equally useful as supporting open vs. non-open classes.

[bioball]

Infer which abstract and open Pkl classes can be sealed in Java/Kotlin.

Without a sealed in Pkl, there's no way to ensure that the provided set of classes is exhaustive. For example:

1. Library publishes Pkl bindings + Java codegen'd classes
2. Application uses library, extends Pkl classes and does is own codegen

[odenix]

I think the conclusion is that moving forward with this feature would require a sealed SPICE.

[bioball]

Yeah; I think that's right

[odenix]

How about reserving the keyword sealed for now?

[bioball]

I'm okay with that!

@stackoverflow, @holzensp: thoughts?