windows: enable process reuse for cmd-shims

[zkat]

As of npx@6.1.0, if the command you're trying to execute turns out to be a Node.js script, npx will do some trickery to "convert" that script into the main module for its own Node process, without spawning a child. This is a pretty significant optimization for what turns out to be a pretty common case for npx users.

Unfortunately, there's no Windows cmd shim detection right now, so this trick works exclusively on *nix platforms.

So, it'd be nice to safely detect the Windows cmd shims that npm installs into $PATH, and grab the Node.js script it's meant to execute -- and call that directly. This should make Windows execution much faster.