Expose Marp Core instance to functional engine

[yhatt]

Improved a functional engine to allow using built-in Marp Core instance by non enumerable marp getter. Importing Marp Core installed by user is no longer required for simple customization.

Example: markdown-it-mark

Previous version

// engine.js
const { Marp } = require('@marp-team/marp-core')
const markdownItMark = require('markdown-it-mark')

module.exports = (opts) => new Marp(opts).use(markdownItMark)

# Install Marp Core and markdown-it-mark
npm install @marp-team/marp-core markdown-it-mark --save-dev

# Specify the path to functional engine
marp --engine ./engine.js slide-deck.md

Current

// engine.js
module.exports = ({ marp }) => marp.use(require('markdown-it-mark'))

# Install markdown-it-mark
npm install markdown-it-mark --save-dev

# Specify the path to functional engine
marp --engine ./engine.js slide-deck.md

A previous way is still available and useful for full-customization.

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In addition, importing Marp Core has put off until needed. It means making faster CLI startup (especially --help).