codegen-kotlin: Don't generate equals/hashCode/toString/copy/:Serializable for abstract classes (#714)

* codegen-kotlin: Don't generate equals/hashCode/toString/copy/:Serializable for abstract classes

* Polish KotlinCodeGeneratorTest

- use `assertContains()` instead of `assertThat().contains()`
- remove redundant `trimIndent()` and `trimMargin()` calls
- rename `javaCode` to `kotlinCode`

* Overhaul KotlinCodeGeneratorTest

- introduce classes KotlinSourceCode and KotlinSourceCodeAssert
- change assertions to use KotlinSourceCodeAssert via `assertThat(kotlinCode)`
- use explicit trimIndent() and trimMargin() for multiline string literals
  - IntelliJ editor desperately wants to insert trimIndent()
  - can be used by kotlinc and ktfmt
- format code in the exact way it falls out of the IntelliJ editor
  (successfully destroyed by ktfmt)

pkl-codegen-kotlin/src/main/kotlin/org/pkl/codegen/kotlin/KotlinCodeGenerator.kt

@@ -485,12 +485,13 @@ class KotlinCodeGenerator(
         builder.addProperty(generateProperty(name, property))
       }
 
-      generateCopyMethods(builder)
-
-      builder
-        .addFunction(generateEqualsMethod())
-        .addFunction(generateHashCodeMethod())
-        .addFunction(generateToStringMethod())
+      if (!pClass.isAbstract) {
+        generateCopyMethods(builder)
+        builder
+          .addFunction(generateEqualsMethod())
+          .addFunction(generateHashCodeMethod())
+          .addFunction(generateToStringMethod())
+      }
 
       return builder
     }
@@ -530,7 +531,7 @@ class KotlinCodeGenerator(
   }
 
   private fun TypeSpec.Builder.ensureSerializable(): TypeSpec.Builder {
-    if (!options.implementSerializable) {
+    if (!options.implementSerializable || modifiers.contains(KModifier.ABSTRACT)) {
       return this
     }