Add "preview" update channel to match macOS

Currently we only have a "stable" update channel. It would be worthwhile to have a preview channel for dogfooding or customers that want to try new features before they're released.

- We will need to generate version numbers for preview builds
- macOS currently does this by setting the 4th version field (`0.0.0.z`) to the number of commits since the last release
- We will need to package them identically to release builds
- And sign them!
- We will need to push the preview artifacts somewhere
- macOS currently does this with a "preview" release, but I dislike this pattern and would probably opt for storing them on `releases.coder.com`
- We will need to update the `SparkleUpdateController` code to handle channel selection, and add UI elements to select a channel

One potential challenge is that our installers forbid downgrading (for good reason). Once a user switches to the preview channel, they won't be able to switch back to the release channel until a new version is released (or they uninstall/reinstall). I think NetSparkle probably handles this for us though if we go with the same version number system as macOS.

Commencez par lire le SparkleUpdateController existant ainsi que le flux actuel de packaging, de signature et de publication des artefacts de release. Définissez comment doivent fonctionner la sélection du canal preview, la numérotation des versions et le stockage sur releases.coder.com, puis vérifiez que les installateurs preview et le retour à stable respectent la contrainte de downgrade indiquée.

Rédigé par le modèle d'indexation à partir du texte de l'issue.