Add SMAPI workflow.

[AlanDavison]

Adds a simple GitHub workflow that:

• Builds SMAPI automatically on push to the develop branch.
• Builds SMAPI automatically when a tag is pushed.
  • Note that this only triggers when tags are in a strict x.y.z format, because it applies SMAPI's beta versioning automatically.
• Uploads installer ZIPs to the run's artifacts.
• Attests installer ZIPs to prove a build came from a given commit.
• Creates a draft GitHub release when a build is started via a tag push.