feat: added keyword codec (partially works)

testdata/data/complex_front_matter/input.md

@@ -0,0 +1,20 @@
+---
+@type: Book
+name: The Great Book
+author:
+  @type: Person
+  name: Jane Doe
+  email: jane.doe@example.com
+tags:
+  - fiction
+  - adventure
+chapters:
+  - name: Chapter 1
+    pages: 20
+  - name: Chapter 2
+    pages: 30
+---
+
+# The Great Book
+
+This is the content of the great book.

testdata/data/complex_front_matter/output.json

@@ -0,0 +1,26 @@
+{
+  "@context": "https://schema.org",
+  "@type": "Book",
+  "name": "The Great Book",
+  "author": {
+    "@type": "Person",
+    "name": "Jane Doe",
+    "email": "jane.doe@example.com"
+  },
+  "tags": [
+    "fiction",
+    "adventure"
+  ],
+  "chapters": [
+    {
+      "name": "Chapter 1",
+      "pages": 20
+    },
+    {
+      "name": "Chapter 2",
+      "pages": 30
+    }
+  ],
+  "headline": "The Great Book",
+  "articleBody": "This is the content of the great book."
+}