Windows version update

[stripedpajamas]

This PR adds support in the update_version.sh script for Windows builds. It means that after deploying new Windows artifacts, the build will cobble together the necessary parts of a version JSON file and commit it to VSCodium/versions for use by the VSCodium/update-api

Changes made:

• Abstract most of the update_version.sh logic into functions because Windows builds have to do everything twice (archive + user)
• Have Windows builds use the node module checksum to compute the sums because shasum doesn't work and Get-FileHash has horrible output
• Made the update commit to /versions instead of hardcoding VSCodium/versions. This makes it so that if you fork VSCodium/versions and add your own GITHUB_TOKEN env var to your fork of VSCodium/vscodium's Travis build, the build will update your own versions repo
• Add shasums for dmg artifacts

cc #41