`std::formatter` specialization

Please provide a `std::formatter` specialization similar to the one `std::stacktrace` provides:

https://godbolt.org/z/x3zKn6vfn

```
terminate called after throwing an instance of 'RuntimeError'
what(): Encountered error code: 139
0# RuntimeError::RuntimeError(std::basic_format_string::type>, int&&) at /app/example.cpp:16
1# h() at /app/example.cpp:23
2# g() at /app/example.cpp:28
3# f() at /app/example.cpp:33
4# main at /app/example.cpp:38
5# at :0
6# __libc_start_main at :0
7# _start at :0
8#
```

Start with the linked Compiler Explorer example and compare its current output with the formatting behavior provided by std::stacktrace. Trace the library's existing stacktrace formatting entry points, then verify that a std::formatter specialization produces equivalent formatted output for the example.

索引モデルが issue の本文から書いたものです。