codegen-java/kotlin: Support generating sealed classes

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.

Start with Java/KotlinCodeGenerator and CliJava/KotlinCodeGenerator, comparing their single-module and multi-module inputs. Determine whether sealedness should be inferred during generation or supported in Pkl itself; done requires an agreed approach and sealed classes generated correctly for eligible hierarchies.

Written by the indexing model from the issue text.