Create a script to automate pushing releases to GitHub

[daviwil]

Needs to do the following things:

1. Run vsce package and unpack to a folder
2. Gather extension files into a folder called ms-vscode.PowerShell-<version>
3. Zip up the extension folder to ms-vscode.PowerShell-<version>.zip
4. Gather the CHANGELOG.md notes for the latest release
5. Create a release using the GitHub API
6. Upload the release ZIP using the GitHub API

[rjmholt]

Closed in #1937