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-commons-test/src/main/kotlin/org/pkl/commons/test/FileTestUtils.kt

@@ -19,6 +19,7 @@ import java.nio.file.Path
 import java.util.stream.Collectors
 import kotlin.io.path.*
 import org.assertj.core.api.Assertions.fail
+import org.opentest4j.AssertionFailedError
 import org.pkl.commons.*
 
 object FileTestUtils {
@@ -110,5 +111,11 @@ data class SnippetOutcome(val expectedOutFile: Path, val actual: String, val suc
   }
 
   private fun failWithDiff(message: String): Nothing =
-    throw PklAssertionFailedError(message, expected, actual)
+    if (System.getProperty("sun.java.command", "").contains("intellij")) {
+      // IntelliJ only shows diffs for AssertionFailedError
+      throw AssertionFailedError(message, expected, actual)
+    } else {
+      // Gradle test logging/report only shows diffs for PklAssertionFailedError
+      throw PklAssertionFailedError(message, expected, actual)
+    }
 }