vim: Cleanup word matching code

**Don't merge it yet!** It requires review from people that actually know the Vim syntax :)
According to the help it has to be `END` (typical use case), or `end`:

                                                            :aug :augroup         
    :aug[roup] {name}               Define the autocmd group name for the         
                                    following ":autocmd" commands.  The name "end"
                                    or "END" selects the default group.