NVIM_LISTEN_ADDRESS may no longer exist

[saep]

I was remined by #100 that the environment variable NVIM_LISTEN_ADDRESS has been removed or changed its meaning. This may mean that nvim-hs no longer automatically connects to neovim on newer versions or the current dev version.

[justinmk]

• Nvim sets $NVIM in child jobs (this is a "passive" variable, child nvim processes don't listen on $NVIM, rather it tells those children the address of the parent).
• For telling nvim itself where to listen, the nvim --listen arg is preferred. For cases where a --listen arg can't be passed to nvim, you can still use $NVIM_LISTEN_ADDRESS.

The reason for the change was that $NVIM_LISTEN_ADDRESS had conflicting purposes which made correct behavior impossible.