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/await/async_concurrent.wren

@@ -0,0 +1,18 @@
+import "scheduler" for Scheduler, Future
+import "timer" for Timer
+
+var order = []
+
+var task1 = async {
+  await Timer.sleep(2)
+  order.add("task1")
+}
+
+var task2 = async {
+  await Timer.sleep(1)
+  order.add("task2")
+}
+
+await task1
+await task2
+System.print(order) // expect: [task2, task1]