feat: add await syntax for direct async function calls and new generator/phonebook apps

Add compiler support for calling async functions directly with `await fn(args)` syntax, automatically translating to `await fn.call(args)`. Introduce `create_merge` Makefile target for merging Wren source files. Include new `apps/generator.wren` source code generator using OpenRouter API, `apps/phonebook.wren` CLI phonebook with SQLite backend, and `apps/minitut.md` Wren tutorial reference. Update `example/await_demo.wren` with parameterized async functions, nested awaits, and expression usage. Fix JSON parsing in `example/openrouter_demo.wren` to strip markdown code fences. Document new await syntax in `manual/api/scheduler.html`.

test/await/await_fn_sequential.wren

@@ -0,0 +1,25 @@
+// retoor <retoor@molodetz.nl>
+
+import "scheduler" for Scheduler, Future
+
+var step1 = async { |x| x + 10 }
+var step2 = async { |x| x * 2 }
+var step3 = async { |x| x - 5 }
+
+var result = 5
+result = await step1(result)
+System.print(result) // expect: 15
+result = await step2(result)
+System.print(result) // expect: 30
+result = await step3(result)
+System.print(result) // expect: 25
+
+var pipeline = async { |x|
+    var r1 = await step1(x)
+    var r2 = await step2(r1)
+    var r3 = await step3(r2)
+    return r3
+}
+
+System.print(await pipeline(0)) // expect: 15
+System.print(await pipeline(10)) // expect: 35