feat: add merge_all_wren utility, strutil demo, and 7 new API manual pages

Add WrenFileReader class in apps/merge_all_wren.wren for recursive .wren file discovery and content printing. Introduce example/strutil_demo.wren demonstrating case conversion, hex encoding, SHA256 hashing, string repetition, padding, HTML/JSON escaping, and URL encoding/decoding. Create manual/api pages for argparse, dataset, html, markdown, uuid, wdantic, and web modules. Update manual/api/index.html sidebar and module count from 21 to 28, adding links and cards for the new modules.

test/dataset/custom_created_at.wren

@@ -0,0 +1,23 @@
+// retoor <retoor@molodetz.nl>
+
+import "dataset" for Dataset
+
+var ds = Dataset.memory()
+var events = ds["events"]
+
+var event = events.insert({
+    "name": "Conference",
+    "created_at": "2024-01-15T10:00:00"
+})
+
+System.print(event["created_at"]) // expect: 2024-01-15T10:00:00
+System.print(event["name"]) // expect: Conference
+
+var found = events.findOne({"name": "Conference"})
+System.print(found["created_at"]) // expect: 2024-01-15T10:00:00
+
+var auto = events.insert({"name": "Meeting"})
+System.print(auto["created_at"] != null) // expect: true
+System.print(auto["created_at"] != "2024-01-15T10:00:00") // expect: true
+
+ds.close()