Add support for const object members (#678)

This adds support for adding the `const` modifier to object members.

Such object members are also required to have the `local` modifier applied.

This follows SPICE-0011.

pkl-core/src/main/java/org/pkl/core/ast/VmModifier.java

@@ -72,7 +72,7 @@ public final class VmModifier {
   public static final int VALID_PROPERTY_MODIFIERS =
       ABSTRACT | LOCAL | HIDDEN | EXTERNAL | FIXED | CONST;
 
-  public static final int VALID_OBJECT_MEMBER_MODIFIERS = LOCAL;
+  public static final int VALID_OBJECT_MEMBER_MODIFIERS = LOCAL | CONST;
 
   public static final int TYPEALIAS_OBJECT_MEMBER = TYPE_ALIAS | CONST;
 

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

@@ -721,12 +721,22 @@ public final class AstBuilder extends AbstractAstBuilder<Object> {
       @Nullable TypeAnnotationContext typeAnnCtx,
       @Nullable ExprContext exprCtx,
       @Nullable List<? extends ObjectBodyContext> bodyCtx) {
-
+    var modifiers =
+        doVisitModifiers(
+            modifierCtxs, VmModifier.VALID_OBJECT_MEMBER_MODIFIERS, "invalidObjectMemberModifier");
+    if (VmModifier.isConst(modifiers) && !VmModifier.isLocal(modifiers)) {
+      @SuppressWarnings("OptionalGetWithoutIsPresent")
+      var constModifierCtx =
+          modifierCtxs.stream().filter((it) -> it.CONST() != null).findFirst().get();
+      throw exceptionBuilder()
+          .evalError("invalidConstObjectMemberModifier")
+          .withSourceSection(createSourceSection(constModifierCtx))
+          .build();
+    }
     return doVisitObjectProperty(
         createSourceSection(ctx),
         createSourceSection(propertyName),
-        doVisitModifiers(
-            modifierCtxs, VmModifier.VALID_OBJECT_MEMBER_MODIFIERS, "invalidObjectMemberModifier"),
+        modifiers,
         propertyName.getText(),
         typeAnnCtx,
         exprCtx,

pkl-core/src/main/resources/org/pkl/core/errorMessages.properties

@@ -19,6 +19,9 @@ Modifier `{0}` is not applicable to properties.
 invalidObjectMemberModifier=\
 Modifier `{0}` is not applicable to object members.
 
+invalidConstObjectMemberModifier=\
+Modifier `const` can only be applied to object members that are also `local`.
+
 objectMethodMustBeLocal=\
 Method needs a `local` modifier because it is defined in an object, not a class.