feat: support ESM plugins

[vicary]

This adds support for the plugins config file to be written in ESM format.

Example usage:

// plugins.js

import MyPlugin from "my-esbuild-plugin";

export default [
  MyPlugin()
];

[github-actions]

🎉 This PR is included in version 1.55.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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