Updater: support one feed name per variant, so two builds of one app can share a release

Currently, the update feed is named after the channel (stable-<platform>-update.json, with the .tar.zst archive next to it). Two variants of one app produce the same feed name, so they cannot be part of the same github release (and share a release.baseUrl).

We package 2 variants of our electrobun desktop apps, a full build and a light build, for Windows x64, Linux, macOS platforms, both portable and installer. They have different app.identifier and I'd love to have all assets of the complete matrix (platforms / app-variant / installer-vs-portable / update-assets) on the same github release. The archive already carries app.name (stable-win-x64-TerrainViewerLight.tar.zst), but update.json does not.

Proposal: an optional release.feedName (or app.name / app.identifier in the feed file's name), used both by electrobun build and by Updater.checkForUpdate.

Workaround: today we publish two GitHub releases desktop-latest and desktop-latest-light

Drafted with AI assistance from Claude Fable