fix(windows): force console codepage to UTF-8 for shell/system() #38742

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()`

(cherry picked from commit bba48ee1b0)

test/functional/vimscript/system_spec.lua

@@ -166,6 +166,11 @@ describe('system()', function()
         test_shell_unquoting()
       end)
 
+      it('spawns child process with UTF-8 codepage', function()
+        command('set shell=cmd.exe')
+        eq('тест\n', eval([[system('echo тест')]]))
+      end)
+
       it('with shell=cmd', function()
         command('set shell=cmd')
         eq('"a b"\n', eval([[system('echo "a b"')]]))