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/table_name.wren

@@ -0,0 +1,16 @@
+// retoor <retoor@molodetz.nl>
+
+import "dataset" for Dataset
+
+var ds = Dataset.memory()
+
+var users = ds["users"]
+System.print(users.name) // expect: users
+
+var products = ds["products"]
+System.print(products.name) // expect: products
+
+var orders = ds["my_orders"]
+System.print(orders.name) // expect: my_orders
+
+ds.close()