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/head_meta.html

@@ -0,0 +1,14 @@
+{# retoor <retoor@molodetz.nl> #}
+<meta charset="UTF-8">
+<meta name="viewport" content="width=device-width, initial-scale=1.0">
+<meta name="description" content="{{ seo.description|default(page_description)|default(site.description) }}">
+<meta name="keywords" content="{{ seo.keywords|default([])|join(', ') }}">
+<meta name="author" content="{{ site.author }}">
+
+<meta property="og:title" content="{{ seo.og_title|default(page_title ~ ' - ' ~ site.name) }}">
+<meta property="og:description" content="{{ seo.description|default(page_description)|default(site.description) }}">
+<meta property="og:type" content="{{ seo.og_type|default('website') }}">
+
+<meta name="twitter:card" content="summary">
+<meta name="twitter:title" content="{{ seo.og_title|default(page_title ~ ' - ' ~ site.name) }}">
+<meta name="twitter:description" content="{{ seo.description|default(page_description)|default(site.description) }}">