Add a new preview release action

[FredKSchott]

Changes

• In the past, this has been super manual. I just saw that Matthew had to deal with this here the other week: Make Astro.url conform to build.format during the build #4352 (comment) as documented here: https://github.com/withastro/astro/blob/main/CONTRIBUTING.md
• Instead, this could be a GitHub action.
  • This PR creates a new action as a "workflow dispatch" event, meaning you have to press a button inside of GitHub's UI to trigger the action.
  • In the future, this could be upgraded to trigger when you comment something like "!preview" directly in a pull request.

Testing

• CI actions are impossible to test in GH until they've been merged to the main branch, so some debugging post-merge may be expected

Docs

• N/A

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[matthewp]

@FredKSchott is this one ready to go (be reviewed)? If not can you switch it to draft? Thanks.

[FredKSchott]

Switched to draft! I think it's basically ready for review but its been off my radar, I'll post in PTAL when its ready for review.

[FredKSchott]

FYI because I want to test this as much as possible before it can actually release packages / talk to npm, here's the process for merging this that I'd like to follow:

• Get approval on this PR as-written
• I'll comment out the actual release step, and then merge. This becomes a dry-run that we can debug in CI
• If the dry-run goes well, Tony will add back the commented-out release step of the CI job in a final PR

[tony-sull]

looks good to me! If you want to comment out the publish step we can test it with a dry-run for @astrojs/image@next