Fix length of listings with computed index (#797)

Motivation:
The following expression evaluates to 2 instead of 1:
new Listing { "value" } { [0 + 0] = "override" }.length

Changes:
- fix length computation in EntriesLiteralNode
- improve `api/listing` tests
- make snippet test failures diffable in IntelliJ

Result:
- fixes https://github.com/apple/pkl/issues/780
- improved dev experience in IntelliJ

pkl-core/src/main/java/org/pkl/core/ast/expression/literal/EntriesLiteralNode.java

@@ -43,15 +43,15 @@ public abstract class EntriesLiteralNode extends SpecializedObjectLiteralNode {
   @Children private final ExpressionNode[] keyNodes;
   private final ObjectMember[] values;
 
-  public EntriesLiteralNode(
+  protected EntriesLiteralNode(
       SourceSection sourceSection,
       VmLanguage language,
-      // contains local properties and default property (if present)
-      // does *not* contain entries with constant keys to maintain definition order of entries
       String qualifiedScopeName,
       boolean isCustomThisScope,
       @Nullable FrameDescriptor parametersDescriptor,
       UnresolvedTypeNode[] parameterTypes,
+      // contains local properties and default property (if present)
+      // does *not* contain entries with constant keys to maintain definition order of entries
       UnmodifiableEconomicMap<Object, ObjectMember> members,
       ExpressionNode[] keyNodes,
       ObjectMember[] values) {
@@ -103,7 +103,8 @@ public abstract class EntriesLiteralNode extends SpecializedObjectLiteralNode {
         frame.materialize(),
         parent,
         createListMembers(frame, parent.getLength()),
-        parent.getLength() + keyNodes.length);
+        // `[x] = y` overrides existing element and doesn't increase length
+        parent.getLength());
   }
 
   @Specialization