refactor: change meta class structure to include types.

Introduce requiredness of translations via implemented interface.

src/core/things/Book.ts

@@ -14,12 +14,13 @@
  */
 import {
   SCCreativeWork,
+  SCCreativeWorkMeta,
   SCCreativeWorkTranslatableProperties,
   SCCreativeWorkWithoutReferences,
 } from '../base/CreativeWork';
 import {SCThingWithCategoriesTranslatableProperties} from '../base/ThingWithCategories';
 import {SCThingMeta, SCThingType} from '../Thing';
-import {SCTranslations} from '../types/i18n';
+import {SCMetaTranslations, SCTranslations} from '../types/i18n';
 import {SCPersonWithoutReferences} from './Person';
 
 /**
@@ -74,20 +75,6 @@ export interface SCBook extends SCCreativeWork, SCBookWithoutReferences {
   type: SCThingType.Book;
 }
 
-/**
- * Meta information about a book
- */
-export class SCBookMeta extends SCThingMeta {
-  static fieldTranslations = {
-    ...SCThingMeta.fieldTranslations,
-    de: {
-      bookEdition: 'Edition',
-      isbn: 'ISBN',
-      numberOfPages: 'Seitenanzahl',
-    },
-  };
-}
-
 /**
  * Translatable properties of a book
  */
@@ -98,3 +85,34 @@ export interface SCBookTranslatableFields
    */
   bookEdition?: string;
 }
+
+/**
+ * Meta information about a book
+ */
+export class SCBookMeta extends SCThingMeta implements SCMetaTranslations<SCBook> {
+  /**
+   * Translations of fields
+   */
+  fieldTranslations = {
+    de: {
+      ... SCCreativeWorkMeta.getInstance().fieldTranslations.de,
+    },
+    en: {
+      ... SCCreativeWorkMeta.getInstance().fieldTranslations.en,
+    },
+  };
+
+  /**
+   * Translations of values of fields
+   */
+  fieldValueTranslations  = {
+    de: {
+      ... SCCreativeWorkMeta.getInstance().fieldValueTranslations.de,
+      type: 'Buch',
+    },
+    en: {
+      ... SCCreativeWorkMeta.getInstance().fieldValueTranslations.en,
+      type: SCThingType.Book,
+    },
+  };
+}