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_in_loop.wren

@@ -0,0 +1,17 @@
+// retoor <retoor@molodetz.nl>
+
+import "scheduler" for Scheduler, Future
+
+var double = async { |x| x * 2 }
+
+var sum = 0
+for (i in [1, 2, 3, 4, 5]) {
+    sum = sum + await double(i)
+}
+System.print(sum) // expect: 30
+
+var results = []
+for (i in 1..3) {
+    results.add(await double(i))
+}
+System.print(results) // expect: [2, 4, 6]