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.v

@@ -24,6 +24,17 @@ pub:
 	// stderr string // TODO
 }
 
+pub struct Command {
+mut:
+	f voidptr
+pub mut:
+	eof       bool
+	exit_code int
+pub:
+	path            string
+	redirect_stdout bool
+}
+
 [unsafe]
 pub fn (mut result Result) free() {
 	unsafe { result.output.free() }