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/tables_property.wren

@@ -0,0 +1,20 @@
+// retoor <retoor@molodetz.nl>
+
+import "dataset" for Dataset
+
+var ds = Dataset.memory()
+
+System.print(ds.tables.count) // expect: 0
+
+ds["users"].insert({"name": "Alice"})
+System.print(ds.tables.count) // expect: 1
+System.print(ds.tables.contains("users")) // expect: true
+
+ds["products"].insert({"name": "Widget"})
+System.print(ds.tables.count) // expect: 2
+System.print(ds.tables.contains("products")) // expect: true
+
+ds["orders"].insert({"total": 100})
+System.print(ds.tables.count) // expect: 3
+
+ds.close()