feat: add wasm build targets, profile hooks, os_demo example, and restructure manual source

- Add `wasm`, `wasm-clean`, and `install-emscripten` phony targets to Makefile for WebAssembly cross-compilation
- Insert `WREN_PROFILE_ENTER`/`WREN_PROFILE_EXIT` macros in `wren_vm.h` and `wren_vm.c` to support optional runtime profiling via `WREN_PROFILE_ENABLED`
- Create `example/os_demo.wren` demonstrating Platform, Process, and conditional exit usage
- Update `example/regex_demo.wren` to import `Match` and exercise Match object properties, groups, and `matchAll`
- Remove static HTML manual pages (`base64.html`, `dns.html`, `json.html`) and replace with structured `manual_src/` directory containing Jinja2 templates, YAML metadata, and content pages
- Expand `README.md` with build targets table, manual building instructions, source layout, and guide for adding new module documentation

example/os_demo.wren

@@ -0,0 +1,36 @@
+// retoor <retoor@molodetz.nl>
+
+import "os" for Platform, Process
+
+System.print("=== OS Module Demo ===\n")
+
+System.print("--- Platform Information ---")
+System.print("Name: %(Platform.name)")
+System.print("Is POSIX: %(Platform.isPosix)")
+System.print("Is Windows: %(Platform.isWindows)")
+System.print("Home Path: %(Platform.homePath)")
+
+System.print("\n--- Process Information ---")
+System.print("PID: %(Process.pid)")
+System.print("Parent PID: %(Process.ppid)")
+System.print("Working Directory: %(Process.cwd)")
+System.print("Wren Version: %(Process.version)")
+
+System.print("\n--- Command Line Arguments ---")
+System.print("Arguments: %(Process.arguments)")
+System.print("All Arguments: %(Process.allArguments)")
+
+System.print("\n--- Platform-Specific Paths ---")
+var separator = Platform.isWindows ? "\\" : "/"
+var configPath = Platform.homePath + separator + ".config"
+System.print("Config directory: %(configPath)")
+
+System.print("\n--- Conditional Exit ---")
+var args = Process.arguments
+if (args.count > 0 && args[0] == "--fail") {
+    System.print("Exiting with error code 1")
+    Process.exit(1)
+}
+
+System.print("Exiting normally")
+Process.exit(0)