Add new GitHub Actions diffcalc spreadsheet generator workflow

[smoogipoo]

I've been running this on my isolated repository (https://github.com/smoogipoo/ga-diffcalc-tester) for a while now, and I want to bring it here so I can use the issue/PR comment functionality :)

It's a first step, and not the last of my work on this.

Further improvements to this:

• Move most of the steps to a shell script as the workflow is hard to read.
• Separate the events into their own workflows, making this a "reusable" one with defined inputs/outputs.
• Run on every tag, comparing against the previous tag.

[peppy]

Not reviewing the content of this, as it doesn't touch the game