Integrate with `vim.ui.open`

[leiserfg]

Nvim now has a vim.ui.open that calls xdg-open or similar, I think gx.nvim should replace it if installed, so any other plugin using that function will automatically get the improvements added by this plugin.

Besides that vim.ui.open has gotten more back-ends (https://github.com/lemonade-command/lemonade) so maybe we should consider wrapping the default implementation, or at least keep the back-ends on sync.

[chrishrb]

yea, i think its time to upgrade this 🥲 will look into this as soon as possible