pkl-config-java: Refine nullness handling in Config and JavaType (#1544)

Motivation:
Config.as() causes nullness warnings when its result is intentionally assigned
to a non-null variable

Changes:

* Introduce Config.asNullable(Class<T>), asNullable(JavaType<T>), and
  asNullable(Type) to explicitly opt into nullable values
* Keep the signatures of Config.as(Class<T>) and Config.as(JavaType<T>)
  unchanged from 0.31 by adding @NullUnmarked
  * This gives users time to migrate from as() to asNullable() where appropriate
  * Avoids introducing new spurious warnings
* Change `<T> T Config.as(Type)` to `<T extends @nullable Object> T Config.as(Type)`
  * This overload is typically used by reflective code such as
    pkl-config-kotlin's Config.to() rather than directly by user code
* Clarify that JavaType<T> represents a non-null top-level type whose type arguments may be nullable
  * Restricting <T> to non-null keeps method signatures understandable for humans and tools
  * Enables full symmetry between Class<T> and JavaType<T> overloads in Config and JavaType
  * Enables future non-null runtime checks in both Config.as() overloads
* Simplify construction of `JavaType`s with nullable type arguments
  * Add ofNullable() variants for most factory methods, e.g., JavaType.listOfNullable()
* Overhaul Javadoc of Config and JavaType

Result:

* Clear separation between accessing nullable and non-null values
* Config.as() is used for the common non-null case
* Config.as() can perform non-null runtime checks in a future release (breaking change)
* More ergonomic construction of types with nullable type arguments
* More detailed and consistent documentation

pkl-config-kotlin/src/main/kotlin/org/pkl/config/kotlin/ConfigExtensions.kt

@@ -40,14 +40,15 @@ import org.pkl.config.kotlin.mapper.KotlinConverterFactories
  *   `as(JavaType.listOf(String::class.java))`
  */
 inline fun <reified T> Config.to(): T {
-  val result = `as`<T>(typeOf<T>().javaType)
-  if (result == null && null !is T) {
-    throw ConversionException(
+  if (null is T) {
+    return asNullable(typeOf<T>().javaType)
+  }
+
+  return `as`(typeOf<T>().javaType)
+    ?: throw ConversionException(
       "Expected a non-null value but got `null`. " +
         "To allow null values, convert to a nullable Kotlin type, for example `String?`."
     )
-  }
-  return result
 }
 
 /**