Upload prebuilt .zip files to the Releases tab

[qrohlf]

Per v4.0.0, the dist folder with build artifacts will no longer be checked in to git. This is a Good Thing, but it also means that users who don't want to use NPM or unpkg will have to build a bundle from source if they want to use Trianglify the old fashioned way.

This ticket is to build a Github Actions workflow that will trigger when a new version tag is pushed, and upload a .zip containing the trianglify.bundle.js and trianglify.bundle.min.js files to the release entry on Github.