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.

example/await_demo.wren

@@ -0,0 +1,22 @@
+// retoor <retoor@molodetz.nl>
+
+import "scheduler" for Scheduler, Future
+import "timer" for Timer
+
+System.print("=== Await Demo ===\n")
+
+System.print("--- Basic Await ---")
+await Timer.sleep(1)
+System.print("Timer completed")
+
+System.print("\n--- Concurrent Async ---")
+var task1 = async { Timer.sleep(1) }
+var task2 = async { Timer.sleep(1) }
+await task1
+await task2
+System.print("Both tasks completed concurrently")
+
+System.print("\n--- Async With Result ---")
+var task = async { "computed value" }
+var result = await task
+System.print("Result: %(result)")