Issue #1912302 by vijaycs85: Create configuration schemas for book module.

core/modules/book/config/schema/book.schema.yml

+# Schema for the configuration files of the book module.
+
+book.settings:
+  type: mapping
+  label: 'Book settings'
+  mapping:
+    allowed_types:
+      type: sequence
+      label: 'Content types allowed in book outlines'
+      sequence:
+        - type: string
+          label: 'Content type'
+    block:
+      type: mapping
+      label: 'Block'
+      mapping:
+        navigation:
+          type: mapping
+          label: 'Navigation'
+          mapping:
+            mode:
+              type: string
+              label: 'Mode'
+    child_type:
+      type: string
+      label: 'Content type for child pages'