Pre-release process improvements

[mrbbot]

#4757 surfaced some ideas people had for improving the workers-sdk prerelease process. This ticket captures them as future quick wins so we don't forget about them. 🙂

• Extract prerelease comment into separate template file (ci: easy pre-releases for new packages #4757 (comment))

• Only include changed packages in prerelease comment (ci: easy pre-releases for new packages #4757 (comment))

• Uploading prereleases for all internal packages, but only including explicitly marked packages in comment (internal chat below)

  By the way, when chatting with Rahul yesterday, we thought it would be interesting to consider only having to mark the pre-releases that you want to have public comments in the PR.

  And then computing all the workspace dependencies and uploading those as artifacts even though they are not marked as prerelease

  Then you don't need devs to worry about adding pre-release marker to every new internal package that might be required by a pre-release external package

  And we avoid polluting the GH PR comments with unnecessary pre-release links

  And ideally in the future, we should see if we could avoid generating external prerelease comments for packages that are not affected by the PR...