Improve the upgrade process for the Twemoji library

[desrosj]

The current process for upgrading the Twemoji library is as follows:

1. The new image assets from the Twemoji release must be uploaded to the wordpress.org CDN. This must be done before any testing occurs. Hitting the URL where a character is expected to be will prime the CDN's cache and should be viewed as having an indefinite lifespan.
   • When only new files/Emoji are added, the previous folder can be used. Since only the new files need to be fetched, it can help to save requests refetching the same asset at a new URL.
   • When old files/Emoji are modified and the modifications change the meaning or how the Emoji is interpreted, a new folder should be added to the CDN. For an example of this, see #51356 and the changes to the kneeling Emoji.
   • When old files/Emoji are modified and the modifications do not change the meaning or interpretation of the character, a new folder is not always necessary.
2. Update the src/js/_enqueues/vendor/twemoji.js file to include the latest changes. There are WP specific changes in this file that must be preserved.
3. The values of baseUrl, svgUrl, and $cdn_url variables in src/wp-includes/formatting.php and $png_cdn/$svg_cdn in tests/phpunit/tests/formatting/emoji.php should be updated to reflect the new paths for the w.org CDN locations.
4. The Twemoji version queried in the replace:emoji-regex Grunt task found in Gruntfile.js should be updated to ensure the correct list of characters is retrieved.
5. Run npm run grunt precommit:emoji to update the list of entities and partials in src/wp-includes/formatting.php.
6. The emoji case within browserSupportsEmoji()` should be updated to test for a character in the most recently supported version of Emoji to ensure all characters are properly displayed.
7. When a new flag is added or adjusted, the conditions used to the browser properly supports the latest version of Emoji in src/js/_enqueues/lib/emoji-loader.js` should be examined.
   • When a new letter locale flag is added, there is a check that should be updated to replace the previous, most recent one (currently Sark as of Emoji 16/[60227]).
   • There may be additional edge cases that warrant new checks. For example, England is a country within the United Kingdom and does not have a two letter locale. It's character sequence is represented by (black flag emoji + [G] + [B] + [E] + [N] + [G]).
   • The transgender flag is the (white flag emoji + transgender symbol) combined with either a ZWJ or zero-width space.
8. The assertions performed in Tests_Formatting_Emoji::test_wp_emoji_list_returns_data are updated to reflect the update being applied.

Other notes:

• The Twemoji library was previously listed as an npm dependency. This changed in [55186] because of the addition of some WordPress specific code to ensure that Emoji replacement does not happen within any element containing the wp-exclude-emoji class.
• Testing an update to Twemoji prior to the CDN can be done using the ​Local Twemoji plugin, or by changing the CDN paths to use the raw content URL from GitHub (ie. ​https://raw.githubusercontent.com/jdecked/twemoji/v17.0.2/assets/72x72/1f004.png provided the release has been tagged, or ​https://raw.githubusercontent.com/jdecked/twemoji/388f518b756404152f128a3539f5e7f9a6f1ebd6/assets/72x72/1f004.png using the full-length SHA value that points to the commit updating the library to the next version).

Some of the steps above can and should be automated using a build task to help prevent mistakes when upgrading the library. For example, logic can be added to the precommit:emoji task to insert the two WordPress specific changes from [55186] into the final version of the file that's shipped in WordPress. Then, the package can be re-added as to the list of dependencies in the package.json file, and a new emoji-upgrade Grunt task could:

• Update the library to the latest version (if there is one).
• Update the version reference in the precommit:emoji Grunt task.
• Perform the precommit:emoji Grunt task.

[desrosj]

I've gone and created a page in ​the Core Handbook that documents the steps listed above for upgrading the Twemoji library.