DIP17/GitHub Actions: on new commits/PR

[philpax]

When new commits are made to a PR in DalamudPluginManifests, including the creation of a new PR, the following should happen:

• grabs the manifest from the PR, as well as the manifest from current HEAD if it exists
• build the project using the action (#873)
• provide an artifact to download that reviewers can easily load into their game
• publish build success/failure to #github
• if an existing plugin, produces a diff between what's currently in HEAD and what's being submitted, and shows it to reviewers (should be able to just use GitHub's diff view between the two commits?)
  • could potentially show the diff between two updates on the same PR, but that requires a bit more work
• the merge is blocked if the build fails
• if it's a testing plugin and the build doesn't fail, it is automatically approved