feat: add emoji

[ntk148v]

[bradfitz]

One bummer thing about this change is that it breaks URLs (the fragments change). Does GitHub Markdown let you specify the fragments explicitly for headings?

[Algoinde]

"The Emojifier"

[ljharb]

The only way to preserve the links is to add an explicit <a id="whatever"></a> directly above the section.

This is probably a good idea anyways, since it ensures that those canonical links won't change even as the titles are edited.

[bradfitz]

While emoji is fun and I'm a fan, I'd like to address stable URLs in a separate PR before we add emoji.