Add lazy loading to images for markdown-it

add-lazy.js

const FIGURE_WITH_LAZY = /<img\s+.*lazy=.*>/g;

const addLazy = (token) => {
	if (token.type === 'image') {
		if (!token.attrs) {
			token.attrs = [];
		}
		token.attrs.push(['loading', 'lazy']);
	} else if (
		token.type === 'html_block' &&
		token.content &&
		token.content.startsWith('<figure>') &&
		!token.content.match(FIGURE_WITH_LAZY)
	) {
		token.content = token.content.replace('<img ', '<img loading="lazy"');
	} else if (token.children) {
		token.children.forEach(addLazy);
	}
}

module.exports = state => state.tokens.forEach(addLazy);