editors/vim, devel/luv: Use luajit-openresty as the Lua backend

After a failed attempt a few months ago, re-enable luajit-openresty
as the default luv backend, and the only neovim backend.

I suspect that it's safe to switch luv unconditionally as well, but I'm
taking a more conservative route at this time and just switching amd64
and i386 to it.

NeoVim is designed specifically for luajit. While it runs on PUC lua,
PUC is not used by upstream and no attempts are made to build for it.

However, luajit is largely a stalled project and is unlikely to ever
see new releases. The OpenResty fork, however, is actively-developed.
It is also the standard neovim lua backend on macOS Homebrew and most
Linux distributions.