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_parallel.wren

@@ -0,0 +1,30 @@
+// retoor <retoor@molodetz.nl>
+
+import "scheduler" for Scheduler, Future
+import "timer" for Timer
+
+var delayedValue = async { |ms, val|
+    Timer.sleep(ms)
+    return val
+}
+
+var f1 = delayedValue.call(10, "first")
+var f2 = delayedValue.call(10, "second")
+var f3 = delayedValue.call(10, "third")
+
+System.print(await f1) // expect: first
+System.print(await f2) // expect: second
+System.print(await f3) // expect: third
+
+var compute = async { |x| x * x }
+
+var futures = []
+for (i in 1..5) {
+    futures.add(compute.call(i))
+}
+
+var results = []
+for (f in futures) {
+    results.add(await f)
+}
+System.print(results) // expect: [1, 4, 9, 16, 25]