Support vim9script files #144

It looks like vimdoc doesn't currently handle vim9script files. As a trivial example, I created

vim9script

##
# Prints a greeting
# @public
def g:SayHello(): void
        echo 'hello world'
enddef

And vimdoc ignored it. Comments use # rather than ". Function definitions are different in Vim 9, as are variable assignments, among other things. It looks like command and mapping definitions are the same.

I don't have an immediate need for this, but if I were to start a new plugin, I would strongly consider using Vim 9 script, rather than legacy vimscript. It looks like there's quite a few vim9 plugins written in the last few years. I don't know what they're using for documentation.