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

manual_src/templates/includes/search_box.html

@@ -0,0 +1,15 @@
+{# retoor <retoor@molodetz.nl> #}
+<div class="search-container" role="search">
+    <input type="text"
+           id="search-input"
+           placeholder="Search... (/)"
+           autocomplete="off"
+           aria-label="Search documentation"
+           aria-autocomplete="list"
+           aria-controls="search-results"
+           aria-expanded="false">
+    <div id="search-results"
+         class="search-dropdown"
+         role="listbox"
+         aria-label="Search results"></div>
+</div>