[V3] Create @apollo/client/core bundle

[kamilkisiela]

This PR creates package.json under /core directory so people are able to use @apollo/client/core module, just like regular node module, with support of bundlers and everything.

Thanks to @apollo/client/core non-react people doesn't have to install react (and @types/react on TypeScript).

It still doesn't fix the issue in codesandbox.io and stackblitz.io and related because in all non-react projects the react module has to be installed... But at least non-react folks are able to use new Apollo Client v3 :)

import { gql, ApolloClient } from '@apollo/client/core';

Related #5532

[wtrocki]

A really important change that is going to fix the main issue for many community packages that will want to migrate to 3.0!

[hwillson]

Thanks for kickstarting this @kamilkisiela. We'll be working on a variation of this over the next little while, and will merge our commits into this PR to keep everything in the same place. We'd love to get your feedback when ready, so we'll definitely keep you posted.

[kamilkisiela]

@hwillson I see you're pushing it forward. Is there a way we could prevent people from using @apollo/client instead of @apollo/client/core in 3rd party libraries, like custom Link and Caches?

I'm talking about not exposing some types and classes in @apollo/client but only in @apollo/client/core so people won't get it wrong. IDEs and their suggestions and auto-imports features would pick the /core and @apollo/client won't leak there.

[abdonrd]

I think it would be much clearer to have something like:

• @apollo/client with just the core
• @apollo/client/react with the React exports

What do you think?

[hwillson]

@abdonrd we considered that in our original design, but decided we want the @apollo/client package to be everything by default, to give people the shortest/easiest possible path to get up and running with Apollo Client. Apollo Client 3 removes apollo-boost, so we want @apollo/client to take its place. While we 💯% want to support people using view layers other than React, React use is still the most popular view option with Apollo, which is why it's part of the full package. There are also other less technical business-y reasons for this decision. Thanks for the feedback though - I'll definitely re-iterate this in our next planning meeting.

@kamilkisiela I'll put some thought into this - thanks!

[benjamn]

Now let's do the same for other entry points (in subsequent PRs)!