How to redirect the output of mimalloc to a file?

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Hi, may I output these mimalloc's infomation to a file?
```
mimalloc: option 'show_errors': 1
mimalloc: option 'show_stats': 1
mimalloc: option 'eager_commit': 1
mimalloc: option 'eager_region_commit': 0
mimalloc: option 'reset_decommits': 0
mimalloc: option 'large_os_pages': 0
mimalloc: option 'reserve_huge_os_pages': 6
...

```

Looks like I can use `mi_register_output`:
```
mi_output_fun(*output_handler) = [](const char* msg, void* arg)
{
// output to a file named "mimalloc.log"
FILE* fp;
if (0 != fopen_s(&fp, "mimalloc.log", "a"))
{
fprintf(fp, "%s\n", msg);
fclose(fp);
}
};
mi_register_output(output_handler, nullptr);
```

But it will open/close the file every time when mimalloc flushes the buffer, which may harm the performance. Could you please provide a demo on how to use it?

Start with the mi_register_output API and the output-buffer behavior described in the issue. Provide a focused usage example for writing mimalloc output to a file without reopening it for every flush; done when the example addresses the performance concern and can be followed by users.

Written by the indexing model from the issue text.