os: add an .exit_code field to os.Command (#13321)

* Added os.Command.exit_code

* vfmt vlib/os/os_test.v

* extract os.Command to os.v, add a dummy panicing implementation on windows, just for parity, fix os_test.v on macos, skip test_command on windows.

Co-authored-by: Merlin Diavova <md@merlindiaova.org>
Co-authored-by: Delyan Angelov <delian66@gmail.com>

vlib/os/os_nix.c.v

@@ -364,16 +364,6 @@ pub fn execute(cmd string) Result {
 	}
 }
 
-pub struct Command {
-mut:
-	f voidptr
-pub mut:
-	eof bool
-pub:
-	path            string
-	redirect_stdout bool
-}
-
 [manualfree]
 pub fn (mut c Command) start() ? {
 	pcmd := c.path + ' 2>&1'
@@ -412,9 +402,9 @@ pub fn (mut c Command) read_line() string {
 	return final
 }
 
-pub fn (c &Command) close() ? {
-	exit_code := vpclose(c.f)
-	if exit_code == 127 {
+pub fn (mut c Command) close() ? {
+	c.exit_code = vpclose(c.f)
+	if c.exit_code == 127 {
 		return error_with_code('error', 127)
 	}
 }