Windows platform support

[cmcgee1024]

Swiftly currently only support Linux and macOS. It could support the Windows platform too.

[cmcgee1024]

Some issues that we expect with Windows that might affect how Swiftly supports it:

• Only a single toolchain can be installed at a time with the current installer
• Symlinks work differently
• C runtime downloads and configuration will require some investigation and may require user documentation for setup
• Trust mechanisms of the binaries work differently

[compnerd]

Only one development snapshot can be installed. However, in theory multiple toolchain releases can be installed in parallel (5.7+). The problem is in the runtime which is not versioned or ABI stable.