Use `#` instead of `"` in Vim9 script

[tyru]

cc: @thinca

Ref. https://vim-jp.slack.com/archives/CLKR04BEF/p1629386264089200

[tyru]

Where Vim9 script is enabled

By @thinca

• Look up :vim9s[cript] at the beginning of file
• Between :def ... :enddef
• command Foo { ... }
• autocmd BufEnter * { ... }

command Foo {
  # Vim9 script
}

autocmd BufEnter * {
  # Vim9 script
}

[obcat]

I've created a pull request #170 to add an initial support for Vim9 script.