Change CDN for Twemoji images from default to jsDelivr

[yhatt]

Related: #320

MaxCDN, Twemoji's default CDN, will shut down at December 31, 2022.
https://www.stackpath.com/blog/maxcdn-and-securecdn-are-retiring-heres-what-it-means-for-you

I had expected the change of default base path in next release of Twemoji, but no longer expected continuous update due to contributor's lay-off and chaos around acquired Twitter.

This PR makes the change to use jsDelivr CDN to provide twemoji images, unless there is specified base path in emoji.twemoji.base constructor option.

// Use Twemoji via jsDelivr
const marp = new Marp();
marp.render(':+1:') // https://cdn.jsdelivr.net/gh/twitter/[email protected]/assets/svg/1f44d.svg

// Use Twemoji via the default CDN (MaxCDN)
const marp = new Marp({
  emoji: {
    twemoji: { base: undefined }
  }
})
marp.render(':+1:') // https://twemoji.maxcdn.com/v/14.0.2/svg/1f44d.svg