feat(typings): describe Kotlin internals and nullability in generated definitions

Moves the dts-generator to the current main, which changes what the generated
definitions describe:

- Kotlin internal classes and members no longer appear. The JVM has no internal
  modifier, so without reading @kotlin.Metadata they were indistinguishable
  from public API. Their names are also mangled with the module they were
  compiled in, which differs between debug and release, so nothing could call
  them across both anyway.
- Reference types known to be nullable are described as unions with null.
  Kotlin metadata states nullability exactly; on the Java side @nullable and
  @nonnull decide, and what carries neither is left as it was rather than being
  claimed non-null. Definitions built from android.jar gain 4,565 null unions
  at compileSdk 35 and change in no other way.
- A class defined by several input jars can be generated from all of them at
  once, for definitions spanning multiple platform jars, where a member a newer
  platform dropped still has to be described. Off unless asked for.

The generator's own command line moved to a double dash spelling, so the task
driving it follows. The single dash spelling still works and is what a packaged
framework from an older release passes; the two ship together, so they cannot
disagree.

Author: edusperoni

test-app/app/build.gradle

@@ -1053,7 +1053,7 @@ task generateTypescriptDefinitions(type: BuildToolTask) {
         // Resolved against the project when building from source; an empty rootPath leaves it
         // relative to workingDir, which is where the packaged framework keeps the jar.
         paramz.add(Paths.get(rootPath, "dts-generator.jar").toString())
-        paramz.add("-input")
+        paramz.add("--input")
 
         for (String jarPath : allJarLibraries) {
             // don't generate typings for runtime jars and classes
@@ -1062,7 +1062,7 @@ task generateTypescriptDefinitions(type: BuildToolTask) {
             }
         }
 
-        paramz.add("-output")
+        paramz.add("--output")
         paramz.add("$TYPINGS_PATH")
 
         new File("$TYPINGS_PATH").mkdirs()