Automatically publish beta releases to make manual testing easier

### Description

Publish unreleased features in beta packages to make it easy to test and iterate on the feature before merging to `main`.

Currently it is difficult to integrate web forms code in Central, including requiring dev environments of both central and web forms which is then difficult to deploy where QA, designers, and user advocates can access it. This adds friction to the feedback loop which slows the process down significantly, and is error prone.

Instead we should automatically publish the build artifact in a way that is easy to integrate with Central or deploy on its own for testing.

Depending on the outcome of https://github.com/getodk/web-forms/issues/535 this could either be an npm package or a docker image using something like GitHub Packages so it doesn't end up mistakenly being used in production. The version of the artifact could be based on the name of the branch which means you always get the latest version of that feature.

### User Stories

- **As a** QA,
**I want** to be able to deploy web-forms features before shipping,
**so that** I can test them thoroughly.

### Dependencies

This should be done after
- https://github.com/getodk/web-forms/issues/636
- https://github.com/getodk/web-forms/issues/535

Start by reviewing issues #636 and #535, which are listed as prerequisites. Then inspect the existing build and release configuration to determine whether the beta artifact should be an npm package or Docker image published through GitHub Packages. Done means a branch-based beta artifact is automatically published and can be integrated with Central or deployed independently for QA testing.

Written by the indexing model from the issue text.