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_closure_capture.wren

@@ -0,0 +1,23 @@
+// retoor <retoor@molodetz.nl>
+
+import "scheduler" for Scheduler, Future
+
+var multiplier = 10
+var scale = async { |x| x * multiplier }
+
+System.print(await scale(5)) // expect: 50
+
+multiplier = 100
+System.print(await scale(5)) // expect: 500
+
+var prefix = "Result: "
+var format = async { |x| prefix + x.toString }
+System.print(await format(42)) // expect: Result: 42
+
+var outer = "outer"
+var fn = Fn.new {
+    var inner = "inner"
+    var capture = async { |x| outer + "-" + inner + "-" + x }
+    return await capture("arg")
+}
+System.print(fn.call()) // expect: outer-inner-arg