PSA: LazyVim now defaults to the snacks picker & explorer for new installs

Like the title says, LazyVim now uses the snacks picker and explorer instead of fzf-lua and neo-tree for new installations of LazyVim. (new as in an existing lazyvim.json does not yet exist).

For existing installations, nothing changes, but users can of course enable the snacks picker/explorer extras to get the same new defaults if they want.