Fix neovim crashes

[svilenb]

By adding a check for vim9script the plugin doesn't crash for neovim or older vim versions

[bfrg]

Since this is a fairly new Vim plugin, I'm assuming everyone has access to a newer Vim version. Vim9script has been out for a few years now.

If you really are using Neovim or a very old Vim version, you can place the plugin under the opt/ directory, like ~/.config/vim/pack/*/opt/vim-qf-diagnostics, where * can be any directory (I use plugins). In your vimrc add:

if has('vim9script')
    packadd vim-qf-diagnostics
endif

For more details, see :h :packadd.