feat: add async/await keywords, Num/String extensions, and Jinja markdown example

Add `async` and `await` token support to the Wren compiler with scheduler resolution logic, extend `Num` with `e` constant, hyperbolic trig, base conversion, and new methods (`isZero`, `gcd`, `lcm`, `digits`, etc.), extend `String` with `lower`, `upper`, `capitalize`, `title`, and regenerate `wren_core.wren.inc`. Include `Makefile` for multi-platform builds, rewrite `README.md` with updated build instructions, and add `example/await_demo.wren` and `example/jinja_markdown.wren` demonstrating new features.

test/jinja/markdown_tags.wren

@@ -0,0 +1,21 @@
+// retoor <retoor@molodetz.nl>
+
+import "jinja" for Environment, DictLoader
+
+var env = Environment.new(DictLoader.new({
+  "md_heading": "{\% markdowntohtml \%}# Hello{\% endmarkdowntohtml \%}",
+  "md_bold": "{\% markdowntohtml \%}**bold**{\% endmarkdowntohtml \%}",
+  "md_var": "{\% markdowntohtml \%}# {{ title }}{\% endmarkdowntohtml \%}",
+  "html_to_md": "{\% markdownfromhtml \%}<h1>Hello</h1>{\% endmarkdownfromhtml \%}",
+  "html_to_md_bold": "{\% markdownfromhtml \%}<p><strong>bold</strong></p>{\% endmarkdownfromhtml \%}",
+  "filter_md": "{{ text|markdown }}",
+  "filter_fromhtml": "{{ html|markdownfromhtml }}"
+}))
+
+System.print(env.getTemplate("md_heading").render({})) // expect: <h1>Hello</h1>
+System.print(env.getTemplate("md_bold").render({})) // expect: <p><strong>bold</strong></p>
+System.print(env.getTemplate("md_var").render({"title": "World"})) // expect: <h1>World</h1>
+System.print(env.getTemplate("html_to_md").render({})) // expect: # Hello
+System.print(env.getTemplate("html_to_md_bold").render({})) // expect: **bold**
+System.print(env.getTemplate("filter_md").render({"text": "**strong**"})) // expect: <p><strong>strong</strong></p>
+System.print(env.getTemplate("filter_fromhtml").render({"html": "<h1>Title</h1>"})) // expect: # Title