ci: mark prerelease versions as GitHub prereleases

[kieronlanning]

Summary

Adds a follow-up job to the Release workflow that flags the GitHub release as a prerelease when package.json carries a prerelease version (for example 2.0.0-prerelease.25).

Why

The release pipeline publishes prerelease versions to NuGet but creates the GitHub release with prerelease: false, so a -prerelease.N release is presented as a stable release on the Releases page. The shared reusable workflow (purview-dev/build) exposes no input for this, so the new job runs after it and applies the flag with gh release edit --prerelease.

Behaviour

• Runs only when the package.json version contains - and a GitHub release for � exists.
• No-ops otherwise: stable versions, and pushes where the tag/release does not exist yet.
• The job requests contents: write; the reusable workflow's own permissions are unchanged.

Validation

• Workflow YAML parses cleanly and the job graph is
  elease -> prerelease.
• Cannot trigger a re-release: the shared workflow skips every release step when �<package.json version> is already tagged.