web/intl: replace homegrown join functions with Intl

[JadedBlueEyes]

This replaces the homegrown list-join functions with react-intl, which uses the browser Intl API under the hood. This is a first step to multilingual support.

This has no user-visible effect at the moment, as the locale is hardcoded to English.

[tulir]

That takes 60kb of js 🙀

[JadedBlueEyes]

Damn. I would expect that it shoud tree-shake just about all of that! The actual code under the hood isn't too complex - we're only really using https://github.com/formatjs/formatjs/blob/main/packages/intl/src/list.ts, so we could depend on that package directly or write our own wrapper over it.

[JadedBlueEyes]

Actual file size diff is -0.06kb in one bundle, +60.34kB in another. Most of this is coming from their ICU messageformat parser, which we don't use here.

Using their provided method to remove that brings that diff down to +20.25 kB. Note that this will require precompiling the strings if we want to keep this setup, see https://formatjs.github.io/docs/guides/advanced-usage/