Revamp release packaging

[dbarnett]

We should reevaluate: What types of packages should we have for each release under https://github.com/google/vimdoc/releases, and how should we build them?

Why: I ran into a few little snags building deb packages for v0.7.1. The instructions at https://github.com/google/vimdoc/wiki/Releasing do still seem to mostly work except that Ubuntu Trusty is no longer supported, so I built for Debian Buster instead. But now I'm wondering for our packaging:

• Do we still want to provide just a deb, or other package formats like Snap, etc? We probably have users on Debian, Ubuntu, OSX, maybe Windows, and automation runners like GitHub Actions.
• Is it worth having some kind of verifiable build chain?
• Can we get automated builds to trigger on each release automatically, or at least reduce some of the manual steps?

[dbarnett]

@snu5mumr1k do you have any thoughts on ideal release packaging?