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/pathlib/parts.wren

@@ -0,0 +1,18 @@
+// retoor <retoor@molodetz.nl>
+
+import "pathlib" for Path
+
+var p1 = Path.new("/home/user/file.txt")
+System.print(p1.parts) // expect: [/, home, user, file.txt]
+
+var p2 = Path.new("relative/path/file.txt")
+System.print(p2.parts) // expect: [relative, path, file.txt]
+
+var p3 = Path.new("")
+System.print(p3.parts) // expect: []
+
+var p4 = Path.new("/")
+System.print(p4.parts) // expect: [/]
+
+var p5 = Path.new("file.txt")
+System.print(p5.parts) // expect: [file.txt]