feat: use ES modules

[JasonBerry]

• removed the reliance on the esm library which has been dead for some time
• updated to Node 18, which is the LTS version still being maintained - Node 12 was EOL'd 2022-04-30
• tried to modify as little as possible to keep the PR small

Fixes #1006

BREAKING CHANGE: code is in in ES Modules now and earliest node support is 18

[Haroenv]

checked that all files are correctly imported with seeing no matches for from '\./.*[^js]';, and runs correctly. thanks!

[Haroenv]

not sure about the history but this likely was an async iife to ensure all errors are caught. Not sure that's needed really tbh. Can be changed in a separate PR and release to be just a regular sync module maybe?

[JasonBerry]

I'm easy with however you want to do it. I was just trying to keep changes to a minimum!

[Haroenv]

why this change by the way? was the formatting not needed in the test?

[JasonBerry]

I don't know if/how this was ever working!

In createPullRequest.js formatPullRequestTitle is never passed to formatPullRequestMessage and so formatPullRequestTitle is undefined and not able to be called. The relevant bit is:

    const title = formatPullRequestTitle({ version: nextVersion, releaseType });
    const message = formatPullRequestMessage({
      repo,
      repoURL,
      baseBranch,
      stagingBranch,
      destinationBranch: baseBranch,
      releaseType,
      diffURL,
      currentVersion,
      nextVersion,
      publishCommands,
      title,
    });

It doesn't make any sense that it was ever working.

[JasonBerry]

This should actually be:

.mockImplementation(({ version, releaseType }) => `# v${version} (${releaseType})`);