Add targets for org.pkl.core.util.Nullable (#235)

Also, change `java.time.@Nullable Duration` to `@Nullable java.time.Duration` to reduce the chance
of false positive errors in IDEs.

pkl-core/src/main/java/org/pkl/core/EvaluatorBuilder.java

@@ -48,7 +48,7 @@ public final class EvaluatorBuilder {
 
   private final Map<String, String> externalProperties = new HashMap<>();
 
-  private java.time.@Nullable Duration timeout;
+  private @Nullable java.time.Duration timeout;
 
   private @Nullable Path moduleCacheDir = IoUtils.getDefaultModuleCacheDir();
 
@@ -343,7 +343,7 @@ public final class EvaluatorBuilder {
   /**
    * Sets an evaluation timeout to be enforced by the {@link Evaluator}'s {@code evaluate} methods.
    */
-  public EvaluatorBuilder setTimeout(java.time.@Nullable Duration timeout) {
+  public EvaluatorBuilder setTimeout(@Nullable java.time.Duration timeout) {
     this.timeout = timeout;
     return this;
   }

pkl-core/src/main/java/org/pkl/core/ast/builder/AstBuilder.java

@@ -2169,7 +2169,7 @@ public final class AstBuilder extends AbstractAstBuilder<Object> {
     return builder;
   }
 
-  private FrameDescriptor.@Nullable Builder createFrameDescriptorBuilder(ObjectBodyContext ctx) {
+  private @Nullable FrameDescriptor.Builder createFrameDescriptorBuilder(ObjectBodyContext ctx) {
     if (ctx.ps.isEmpty()) return null;
 
     checkCommaSeparatedElements(ctx, ctx.ps, ctx.errs);

pkl-core/src/main/java/org/pkl/core/project/Project.java

@@ -67,7 +67,7 @@ public final class Project {
   public static Project loadFromPath(
       Path path,
       SecurityManager securityManager,
-      java.time.@Nullable Duration timeout,
+      @Nullable java.time.Duration timeout,
       StackFrameTransformer stackFrameTransformer,
       Map<String, String> envVars) {
     try (var evaluator =
@@ -90,7 +90,7 @@ public final class Project {
 
   /** Convenience method to load a project with the default stack frame transformer. */
   public static Project loadFromPath(
-      Path path, SecurityManager securityManager, java.time.@Nullable Duration timeout) {
+      Path path, SecurityManager securityManager, @Nullable java.time.Duration timeout) {
     return loadFromPath(
         path, securityManager, timeout, StackFrameTransformers.defaultTransformer, System.getenv());
   }

pkl-core/src/main/java/org/pkl/core/util/Nullable.java

@@ -20,7 +20,13 @@ import static javax.annotation.meta.When.MAYBE;
 import java.lang.annotation.*;
 import javax.annotation.meta.TypeQualifierNickname;
 
-@Target({ElementType.TYPE_PARAMETER, ElementType.TYPE_USE})
+@Target({
+  ElementType.TYPE_PARAMETER,
+  ElementType.TYPE_USE,
+  ElementType.PARAMETER,
+  ElementType.FIELD,
+  ElementType.METHOD
+})
 @Retention(RetentionPolicy.RUNTIME)
 @Documented
 @javax.annotation.Nonnull(when = MAYBE)