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_already_resolved.wren

@@ -0,0 +1,8 @@
+import "scheduler" for Scheduler, Future
+import "timer" for Timer
+
+var task = async { "ready" }
+
+await Timer.sleep(10)
+System.print(task.isDone) // expect: true
+System.print(await task)  // expect: ready

test/await/async_basic.wren

@@ -0,0 +1,7 @@
+import "scheduler" for Scheduler, Future
+import "timer" for Timer
+
+var task = async { 42 }
+
+var result = await task
+System.print(result) // expect: 42

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]

test/await/basic.wren

@@ -0,0 +1,5 @@
+import "scheduler" for Scheduler
+import "timer" for Timer
+
+await Timer.sleep(1)
+System.print("done") // expect: done

test/await/in_method.wren

@@ -0,0 +1,11 @@
+import "scheduler" for Scheduler
+import "timer" for Timer
+
+class Helper {
+  static run() {
+    await Timer.sleep(1)
+    return "completed"
+  }
+}
+
+System.print(Helper.run()) // expect: completed

test/await/return_value.wren

@@ -0,0 +1,6 @@
+import "scheduler" for Scheduler
+import "timer" for Timer
+
+var result = await Timer.sleep(1)
+System.print(result == null) // expect: true
+System.print("done") // expect: done