Add packageManager key to core/package.json now we recommend corepack enable

This is even more important when corepack becomes manadatory for developing on multiple core branches following 📌 Update from Yarn 1 to Yarn 4 RTBC

While I couldn't reproduce this locally (I am on an older NodeJS/corepack setup than Alex) I think this is a good idea:

• Yarn will apparently warn if packageManager is set but corepack is not enabled, which will help users get it installed locally.
• Once corepack is enabled it means everyone will use the specified version of yarn locally.

Committing because I cannot commit to Drupal until this is merged... lol

Committed and pushed ef8b6e30b8 to 11.x and 2c9040ee8f to 10.3.x and c13f63ce92 to 10.2.x. Thanks!

Automatically closed - issue fixed for 2 weeks with no activity.

This broke my commit workflow and unfortunately took a bit of head-desking to figure out what was going on; I spent quite a bit of time tearing apart and rebuilding my toolchain before I understood the problem. From the yarn page about corepack:

DANGER
Some third-party distributors may not include Corepack by default, in particular if you install Node.js from your system package manager. If that happens, running npm install -g corepack before corepack enable should do the trick.

I think this needs a change record and probably a handbook page update somewhere. I already updated https://github.com/alexpott/d8githooks/blob/main/README.md but that's not what 99.99% of people using core's build tooling locally will reference. :)

In terms of change records we can probably just add to the main Yarn 4 one at https://www.drupal.org/node/3428571; → doing that now.

Updated the CR; still needs to go in the handbook setup instructions somewhere though.