ci/github-script/prepare: init from actions/get-merge-commit

This just moves the code over to ci/github-script to make it easy to
test and iterate on locally.

The name `prepare` is chosen, because the script will be extended with
the other steps from "PR / prepare" next.

ci/github-script/run

@@ -39,6 +39,17 @@ async function run(action, owner, repo, pull_number, dry = true) {
   })
 }
 
+program
+  .command('prepare')
+  .description('Prepare relevant information of a pull request.')
+  .argument('<owner>', 'Owner of the GitHub repository to check (Example: NixOS)')
+  .argument('<repo>', 'Name of the GitHub repository to check (Example: nixpkgs)')
+  .argument('<pr>', 'Number of the Pull Request to check')
+  .action(async (owner, repo, pr) => {
+    const prepare = (await import('./prepare.js')).default
+    run(prepare, owner, repo, pr)
+  })
+
 program
   .command('commits')
   .description('Check commit structure of a pull request.')