Add Windows support

[TimMensch]

Currently mdx-site doesn't work at all on Windows.

I tweaked package.json and the defaults to make things work.

I also added a default npm_package_bin_mdx_site path (which must be set in your environment?), and I bumped a few versions of packages that were causing problems on my system (possibly because I have the latest TypeScript? possibly because Windows? Don't know).

This is likely to be my only contribution because I ran into a wall: I can't import mdx or md files into other mdx files, or at least I haven't been able to figure out how to. This is a showstopper, and I am going to jump to another solution (Next.js is my current plan) that does less work for me, but that is more flexible. But since I'd done the work I thought I'd throw it over the wall in case it's useful to you or others.