Include node.exe with the Windows release of Sapling.

[bolinfest]

On Ubuntu and macOS, we use package managers (apt, brew) to install Sapling, which makes it straightforward to depend on the platform's latest Node.js package so we don't have to vendor it into Sapling itself. Unfortunately, this is not the case on Windows, which creates a speedbump for users when trying to get sl web to work for the first time.

There are various tools to wrap up a Node.js project into a Windows .exe:

• https://github.com/vercel/pkg
• https://github.com/nexe/nexe

We should use one of these to package up the executable behind sl web on Windows and include it with Sapling so end-users do not have to install their own Node.js