Split out types from @formatjs/intl-displaynames into a separate package

[ayhernandez-godaddy]

For context, my team is a bit sensitive about our project's filesystem usage because the smaller we can keep our Docker image, the faster our builds and deploys tend to be. Our project's node_modules directory is usually the biggest contributor to disk usage, so we sometimes investigate if there are any opportunities to slim down our dependencies.

I noticed that on react-intl@5.20.10, its total disk space usage (including all of its dependencies) is 15MB. I decided to investigate why that is, and found that the dependency on @formatjs/intl-displaynames is itself 14MB. As far as I can tell from a code search, @formatjs/intl-displaynames is only imported in two locations:

1. https://github.com/formatjs/formatjs/blob/main/packages/intl/src/displayName.ts#L3-L6
2. https://github.com/formatjs/formatjs/blob/main/packages/react-intl/index.ts#L17

Both of these imports appear to be used only for types. As I understand it, @formatjs/intl-displaynames is supposed to be an optional dependency that is only needed to polyfill Intl.DisplayNames, and thus the actual code of that package isn't required by react-intl.

I want to know if the maintainers would be open to the idea of splitting out the types into a separate package. Then, the bulk of the 14MB code can remain an optional polyfill dependency that doesn't need to be included in a project's filesystem that doesn't require that polyfill.

[longlho]

so the real fix here is to add Intl.DisplayNames to https://github.com/microsoft/TypeScript/blob/main/src/lib/es2021.intl.d.ts but I haven't got a chance to work on that yet. Once that's in we don't need the types from the polyfill anymore.