codegen-java: Change condition for generating equals/hashCode/toString/with/Serializable (#710)

Motivation:
- Currently, the condition for generating equals/hashCode/toString methods is "generated class declares properties".
  As a result, classes that don't declare properties inherit these methods from their generated superclasses.
  However, generated `equals` and `toString` methods aren't designed to be inherited
  and will either fail or produce wrong results when called for a subclass.
- Currently, the condition for generating `with` methods is "class is not abstract".
  However, it isn't useful to generate `with` methods for non-instantiable non-abstract classes.
- Currently, the condition for making classes serializable is "class is not a module class".
  However, it isn't useful to make non-instantiable non-module classes serializable,
  and it is useful to make instantiable module classes serializable.

Changes:
- Change condition for generating equals/hashCode/toString/with/Serializable to "class is instantiable".
  This is a breaking change.
  (A generated class is instantiable, i.e., declares a public constructor,
  if it is neither abstract nor stateless. This behavior remains unchanged for now.)
- Overhaul JavaCodeGeneratorTest
  - introduce classes JavaSourceCode and JavaSourceCodeAssert
  - change assertions to use JavaSourceCodeAssert via `assertThat(javaCode)`
  - use parameterized test instead of loop
  - use explicit trimIndent() and trimMargin() for multiline string literals
    - IntelliJ editor desperately wants to insert trimIndent()
    - can potentially be exploited by kotlinc and ktfmt

Result:
- Fixes all motivating issues.
- Fixes #706.

pkl-codegen-java/src/test/resources/org/pkl/codegen/java/Inheritance.jva

@@ -32,13 +32,23 @@ public final class Mod {
     public long getOne() {
       return one;
     }
+  }
+
+  public static class None extends Foo {
+    public None(@Named("one") long one) {
+      super(one);
+    }
+
+    public None withOne(long one) {
+      return new None(one);
+    }
 
     @Override
     public boolean equals(Object obj) {
       if (this == obj) return true;
       if (obj == null) return false;
       if (this.getClass() != obj.getClass()) return false;
-      Foo other = (Foo) obj;
+      None other = (None) obj;
       if (!Objects.equals(this.one, other.one)) return false;
       return true;
     }
@@ -53,23 +63,13 @@ public final class Mod {
     @Override
     public String toString() {
       StringBuilder builder = new StringBuilder(100);
-      builder.append(Foo.class.getSimpleName()).append(" {");
+      builder.append(None.class.getSimpleName()).append(" {");
       appendProperty(builder, "one", this.one);
       builder.append("\n}");
       return builder.toString();
     }
   }
 
-  public static class None extends Foo {
-    public None(@Named("one") long one) {
-      super(one);
-    }
-
-    public None withOne(long one) {
-      return new None(one);
-    }
-  }
-
   public static class Bar extends None {
     protected final String two;