restore 'Not Implemented' for legacy script commands (:lua :ruby ...)

- restores behavior equivalent to Vim compiled without FEAT_MZSCHEME, etc.
  (avoids spurious "E492: Not an editor command: EOF") for code such as:
```vim
  if 0
    perl << EOF
  end
  EOF
  endif
```
  see :help script-here
- fixes #1308
- fixes https://github.com/junegunn/vim-plug/issues/111
Merge pull request #1747 from justinmk/ex_ni

restore 'Not Implemented' for legacy script commands (:lua :ruby ...)