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/home.html

@@ -0,0 +1,18 @@
+{# retoor <retoor@molodetz.nl> #}
+{% extends 'base.html' %}
+{% from 'macros/components.html' import card_grid, module_grid %}
+
+{% block main %}
+    <div class="hero">
+        <h1>{{ site.name }} Manual</h1>
+        <p>{{ site.description }}</p>
+        <div class="hero-buttons">
+            <a href="{{ static_prefix }}getting-started/index.html" class="primary-btn">Get Started</a>
+            <a href="{{ static_prefix }}api/index.html" class="secondary-btn">API Reference</a>
+        </div>
+    </div>
+    <article>
+        {% block article %}{% endblock %}
+    </article>
+    {% include 'includes/page_footer.html' %}
+{% endblock %}