Leverage basic Java 17 features (#451)

- Refactor code to use the following basic Java 17 features:
  - pattern matching for instanceof
  - @Serial annotation
  - switch expressions
  - enhanced switch statements
  - StringBuilder.isEmpty()
- Replace two switch statements with simpler if statements.
- Rename a few local variables.

pkl-executor/src/main/java/org/pkl/executor/ExecutorOptions.java

@@ -454,37 +454,36 @@ public final class ExecutorOptions {
   }
 
   ExecutorSpiOptions toSpiOptions() {
-    switch (spiOptionsVersion) {
-      case -1:
-      case 2:
-        return new ExecutorSpiOptions2(
-            allowedModules,
-            allowedResources,
-            environmentVariables,
-            externalProperties,
-            modulePath,
-            rootDir,
-            timeout,
-            outputFormat,
-            moduleCacheDir,
-            projectDir,
-            certificateFiles,
-            certificateUris,
-            testPort);
-      case 1: // for testing only
-        return new ExecutorSpiOptions(
-            allowedModules,
-            allowedResources,
-            environmentVariables,
-            externalProperties,
-            modulePath,
-            rootDir,
-            timeout,
-            outputFormat,
-            moduleCacheDir,
-            projectDir);
-      default:
-        throw new AssertionError("Unknown ExecutorSpiOptions version: " + spiOptionsVersion);
-    }
+    return switch (spiOptionsVersion) {
+      case -1, 2 ->
+          new ExecutorSpiOptions2(
+              allowedModules,
+              allowedResources,
+              environmentVariables,
+              externalProperties,
+              modulePath,
+              rootDir,
+              timeout,
+              outputFormat,
+              moduleCacheDir,
+              projectDir,
+              certificateFiles,
+              certificateUris,
+              testPort);
+      case 1 -> // for testing only
+          new ExecutorSpiOptions(
+              allowedModules,
+              allowedResources,
+              environmentVariables,
+              externalProperties,
+              modulePath,
+              rootDir,
+              timeout,
+              outputFormat,
+              moduleCacheDir,
+              projectDir);
+      default ->
+          throw new AssertionError("Unknown ExecutorSpiOptions version: " + spiOptionsVersion);
+    };
   }
 }

pkl-executor/src/main/java/org/pkl/executor/Version.java

@@ -198,9 +198,7 @@ final class Version implements Comparable<Version> {
   @Override
   public boolean equals(/* @Nullable */ Object obj) {
     if (this == obj) return true;
-    if (!(obj instanceof Version)) return false;
-
-    var other = (Version) obj;
+    if (!(obj instanceof Version other)) return false;
     return major == other.major
         && minor == other.minor
         && patch == other.patch