Support ES Modules and async resolution for engine script

[yhatt]

--engine option now supports using ES Modules and async function engine.

ESM

// engine.mjs
import { marpPlugin } from 'marp-plugin-foobar';
export default ({ marp }) => marp.use(marpPlugin);

marp --engine engine.mjs markdown.md

Async function

with ES Modules

// engine.mjs
export default async ({ marp }) => {
  const { marpPlugin } = await import('marp-plugin-foobar');
  return marp.use(marpPlugin);
}

marp --engine engine.mjs markdown.md

with CommonJS

// engine.cjs
module.exports = async ({ marp }) => {
  const { marpPlugin } = await import('marp-plugin-foobar');
  return marp.use(marpPlugin);
}

marp --engine engine.cjs markdown.md

Backward compatibillity

In the most cases, a formerly used engine.js with CommonJS should keep working.

// engine.js
module.exports = ({ marp }) => marp.use(require('marp-plugin-foobar').marpPlugin)

marp --engine engine.js markdown.md

However, if a Node project that Marp CLI was installed has been marked as type: module in package.json, users may have to change the engine file extension from .js to .cjs for making clear of the intent of using CommonJS.

Notes

• Removed boot optimization by v8-compile-cache due to incompatibillity with ES Modules. Related: v2.2.1 leads to segmentation fault (core dumped) #487