Pre-sanitize the command before handling

- Use a global pointer for the current sanitized command
- Pre-sanitize the current command to bypass `N` and nullify `*`,
removing the need for handlers to bypass, ignore, or nullify these
parts, and reducing overhead for `code_seen`, etc.
- Pre-skip leading whitespace.
- Only look for G, M, T codes at the start of the command.
- Verify that G, M, T codes are followed by command codes.