feat: Determine server info

[mikew]

Part of what's holding back Windows report support is that nvrh assumes a bash/unix like environment.

Most of it isn't so bad, it expects /tmp to exist, but the main command to start the remote neovim instance relies on a fairly bash-compatiable shell, since it uses $SHELL -i -c '...'

This PR adds some groundwork for determining some server info (namely the shell and temp directory), but it's a chicken an egg problem, since it relies on starting the remote neovim instance before it can begin doing that.

I think there's maybe some path forward where we can just run nvim -u NONE --headless -listen :SOME_PORT

• ports because they work on both Windows and Unix
• -u NONE because we're just going to run some lua

But this expects nvim to exist in the users PATH, which is why I've been using $SHELL -i because ... guess who has two thumbs and nvim isn't in my PATH until fully loading an interactive shell ...