Tweak structure of the plugins repository

The plugin installation code expects the manifest for the latest version of the plugin to be at zarjazz.json, with previous versions at zarjazz@1.4.0.json, zarjazz@1.3.0.json, etc.

A slightly vexing side effect of this is that in order for a plugin developer to update a plugin, they must move zarjazz.json to zarjazz@1.5.0.json and then put the new latest in its place. This feels at odds with what feels like a purely additive operation.

It also has the vexing side effect for reviewers of not seeing a clean diff with the new file. Instead they see the old version as a new file, with diffs for URLs, digests, etc. in the "current version" file. It is therefore tricky when reviewing to verify that the old version hasn't changed.

It would be good to tweak the plugin code to allow new versions to be added to the plugins repo in as purely additive a way as possible, with any diffs to existing files being minor and easily verifiable by eye. For example, we could version all JSON files (so that the new one comes in as zarjazz@1.6.0.json instead of zarjazz.json). And maybe we'd need a current.txt file to point to current but that would be easy to check, or maybe we could make the installer super duper smart enough to figure it out, or something.

Obviously there are ghastly compatibility implications for this.

Anyway throwing this out for consideration and discussion. (And doing it here because it's really about the Spin code, the plugins repo just follows what that dictates.)

Start by tracing the plugin installation code and how it locates the latest and previous version manifests in the plugins repository. Review the compatibility implications and the proposed alternatives before choosing an approach; done means new plugin versions can be added mostly additively without breaking existing installations.