Add --check option, for CI

GitHub recommends ncc for building actions for GitHub Actions. When making a change to the repository for an existing action, it's very easy to forget to re-run ncc, so that the generated code is updated.

I would like to build an action that can verify that the code in the repository matches the generated code from ncc. To do that, it would be convenient to have a ncc --check option, which would compile the code in the repo, compare the result to the generated file that already exists, and exit with status code 0 if they match, or status code 1 if they do not match. This could be turned into an action to run on repositories for other actions, to make sure that code changes include regenerating the code.

Start at the ncc command-line entry point and inspect how compilation writes the generated bundle. Compare the generated output with the repository's existing file and verify the requested exit statuses; the issue names no specific file or test, so those must be located in the project.