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_windows.c.v

@@ -543,3 +543,17 @@ pub fn getegid() int {
 pub fn posix_set_permission_bit(path_s string, mode u32, enable bool) {
 	// windows has no concept of a permission mask, so do nothing
 }
+
+//
+
+pub fn (mut c Command) start() ? {
+	panic('not implemented')
+}
+
+pub fn (mut c Command) read_line() string {
+	panic('not implemented')
+}
+
+pub fn (mut c Command) close() ? {
+	panic('not implemented')
+}