Draft pull request for Windows support.

This is still early in the exploration phase, anyone wanting to contribute or follow the specific implementation details should subscribe to this thread.

More general Windows support discussion can be found on the original issue #22

List of changes so far to get server working:-

• Use Windows specific environment variable setup for the yarn win-dev server launch script
• Call file with double quotes instead of single quotes (the Windows version of file needs to be installed, see below)
• Strip leading backslashes \ from file paths, like we do for forward slashes / (having backslashes after that is ok, browsers in Windows are setup to expect that).
• Add C: back on when calling out to file

These early tests come with the following limitations:-

• You can't use shortened home paths (~)
• You can't access anything but C:\
• You need to install the Windows version of the file command and add it to your Windows PATH environment variable
• I haven't looked at the viv client at all yet, so you have to manually invoke the browser