After T133462 is resolved, we can split some of the mediawiki.base utilities into their own file and publish them as their own versioned libraries.
* mediawiki.html (mw.html.element, mw.html.escape)
* mediawiki.message (mw.Message, mw.msg)
>>! **Original task description:**
> I'd like to rectify this with the following proposal:
>  Introduce mediawiki-map shim npm library for browsers that do not support Map
>  Introduce mediawiki-html-construction-helper npm library (https://github.com/jdlrobson/mediawiki-html-construction-helper)
>  Introduce mediawiki-messages module that makes use of browser native Map and mediawiki-html-construction-helper (https://github.com/jdlrobson/mediawiki-messages)
>  Update StartupModule to load these libraries
>  Publish libraries on npm for consumption outside the MediaWiki ecosystem.
> * Removal of Map code should save us a few bytes (browser support is pretty good for what we use it for)
> * More generic reusable code for other people's projects.