Add link to Chromatic build on PRs

[PatrickDeVries]

This may require updating CI to use the Chromatic GitHub Action.
Adding a build link to the PR is not yet supported directly but the build link is output by the Chromatic GitHub action and could then be added as a normal comment.
Only the latest build should be linked, so the comment should be updated when it already exists rather than adding a new one.

[PatrickDeVries]

Because actions get a read-only token on PRs form forks, this cannot be done for PRs from forks without security risks / a separate server with a PAT, so we cannot do this. Instead, we can just build it in main on merge and output the build info to the action output.