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/markdown/blocks_advanced.wren

@@ -0,0 +1,22 @@
+// retoor <retoor@molodetz.nl>
+
+import "markdown" for Markdown
+
+var multiLine = Markdown.toHtml("```\nline 1\nline 2\nline 3\n```")
+System.print(multiLine.contains("line 1")) // expect: true
+System.print(multiLine.contains("line 2")) // expect: true
+System.print(multiLine.contains("line 3")) // expect: true
+System.print(multiLine.contains("<pre><code>")) // expect: true
+
+var multiQuote = Markdown.toHtml("> line 1\n> line 2")
+System.print(multiQuote.contains("<blockquote>")) // expect: true
+System.print(multiQuote.contains("line 1")) // expect: true
+System.print(multiQuote.contains("line 2")) // expect: true
+
+var withLang = Markdown.toHtml("```wren\nSystem.print(\"hello\")\n```")
+System.print(withLang.contains("<pre><code>")) // expect: true
+System.print(withLang.contains("System.print")) // expect: true
+
+var emptyBlock = Markdown.toHtml("```\n\n```")
+System.print(emptyBlock.contains("<pre><code>")) // expect: true
+System.print(emptyBlock.contains("</code></pre>")) // expect: true