pkl-core: Migrate nullness to JSpecify (#1601)

Replace pkl-core's local nullness annotations with JSpecify annotations.
Enable NullAway checking for pkl-core packages except org.pkl.core.ast
and org.pkl.core.stdlib.

Notable code changes:
- Add a dedicated late-init constructor to VmTyped
- Move VmExceptionBuilder's fallback message derivation from withCause()
to build()
- Split VmException rendering between builder-provided messages and
string-backed messages
- Initialize MessageTransport handlers with default throwing handlers
- Update JSON helper collection types to allow nullable values JSON
arrays and objects can contain JSON null,
so the Java Map/List element types need to model nullable elements
explicitly
- Make public command transform APIs accept nullable transformed values 
   Command transforms can produce null for optional/default handling,
so the BiFunction and options-map element types now model that
explicitly
- Make ExecutorSpiException accept nullable message and cause 
Existing call sites can pass nullable causes from Throwable.getCause()
- Remove JSR-305 semantics from `@LateInit`
   JSpecify does not support the same type-qualifier-nickname pattern,
so `@LateInit` is now documentation plus a NullAway
constructor-initialization exemption

Out of scope:
- NullAway checking of org.pkl.core.ast and org.pkl.core.stdlib
- IntelliJ warnings related to `@LateInit` fields
- Removing the JSR-305 dependency, since concurrency annotations are
still in use

pkl-core/pkl-core.gradle.kts

@@ -13,6 +13,8 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+import net.ltgt.gradle.errorprone.errorprone
+import net.ltgt.gradle.nullaway.nullaway
 import org.apache.tools.ant.filters.ReplaceTokens
 
 plugins {
@@ -21,6 +23,7 @@ plugins {
   id("pklJavaLibrary")
   id("pklPublishLibrary")
   id("pklNativeLifecycle")
+  id("pklJSpecify")
   idea
 }
 
@@ -57,6 +60,7 @@ dependencies {
 
   add("generatorImplementation", libs.javaPoet)
   add("generatorImplementation", libs.truffleApi)
+  add("generatorImplementation", libs.jspecify)
 
   javaExecutableConfiguration(project(":pkl-cli", "javaExecutable"))
 }
@@ -107,6 +111,18 @@ tasks.processResources {
 
 tasks.compileJava { options.generatedSourceOutputDirectory.set(file("generated/truffle")) }
 
+tasks.withType<JavaCompile>().configureEach {
+  options.errorprone.nullaway {
+    // Do not require LateInit fields to be initialized at construction time.
+    // Unfortunately, IntelliJ doesn't currently understand this,
+    // and therefore emits many warnings related to LateInit.
+    excludedFieldAnnotations.add("org.pkl.core.util.LateInit")
+
+    // For now, don't analyze code that deals with Truffle ASTs.
+    unannotatedSubPackages.addAll("org.pkl.core.ast", "org.pkl.core.stdlib")
+  }
+}
+
 tasks.compileKotlin { enabled = false }
 
 tasks.test {