Emoji plugin: Allow using other emoji vendors

[yhatt]

Marp Core's emoji plugin converts Unicode 14.0 emoji into the vector image elements provided by Twemoji (Twitter emoji), for getting better emoji rendering in PDF and printing.

In every year, we have kept update to catch up the latest Unicode as usual. However, the latest Twemoji (Unicode 15 compliant) is not yet released due to confusions around Twitter brought by Elon's acquisition. It's doubtful whether the next Twemoji update will come in this year.

Marp relys on Twemoji since 6 years, but it may be a time for getting support other emoji variants.

Plans

• Marp class should provide helper methods, to render emoji elements and styles for them, just as like as Marp.prototype.highlighter().

  • Plugin author and engine developer can overload these (protected) methods, to swap emoji rendering into images that are provided by the other vendor.
  • Class method interfaces may be better to consider the case using emoji through Web fonts (COLRv0/v1).
  • Marp team does not take this approach by default, to avoid an initial loading of huge emoji assets.

• Marp Core v3 is going to keep Twemoji rendering in emoji for Unicode 14.0.

  • [CHANGE] MaxCDN, the Twemoji's default CDN for delivering emoji images, will shut down at the end of 2022. So we have to provide a proper CDN as alternate default (e.g. jsDelivr) -> #321

[yhatt]

The forked version of Twemoji by an original author has been updated to v14.1.1. It still has a blocker about TypeScript compatibillity (https://github.com/jdecked/twemoji/pull/1), but at least we should switch Twemoji to the well-maitained version for supporting Unicode 15 emojis in future.

[yhatt]

Unicode 15 support for a forked version of Twemoji has been announced. This proposal was created in response to concerns about the discontinuation of Twemoji's ongoing maintenance, so we may not need to worry about it for now.

[yhatt]

UPDATE: The blocker about TypeScript (https://github.com/jdecked/twemoji/pull/1) has been removed in Twemoji v15.1 released yesterday. :smile: