Implement compile-time format attribute parsing

[wjjsn]

Summary

• add compile-time analysis of format strings to build attribute tables for literals, escapes, and placeholders
• route argument printing through the analyzed attributes with support for integral, floating, and string types
• update the example program to exercise placeholder and escaped brace handling

Testing

• cmake --build build
• ./build/em_fmt

---

https://chatgpt.com/codex/tasks/task_b_68f823a21a74832d96dbc52440b11271

[wjjsn]

1.write_argument应该使用concept或模板特化以支持对自定义类型的支持
2.打印float时不应该申请这么大的栈内存。
3.应该添加{:xx}格式解析