Problem:
On Windows, `:!echo тест` shows `????` because the console code page defaults to a legacy ANSI encoding (e.g. CP1252) instead of `UTF-8`
Solution:
Call `SetConsoleOutputCP(CP_UTF8)` and `SetConsoleCP(CP_UTF8)` in `do_os_system()` before spawning child processes, and restore the original values after. It covers both `:!` and `system()` since they both go through `do_os_system()`