[RDY] 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
:lu is  an abbreviation for :lunmap, so this block should be inserted after the `'lunmap'` command. Otherwise
:lu will give error `E319: Sorry, the command is not available in this version` instead of `E474: Invalid argument`.

I apply this change to vim-patch: 7.4.211 ( https://github.com/neovim/neovim/pull/521)