[pjson] support Corepack's `"packageManager"` field

[DerekNonGeneric]

• [x] I'd be willing to implement this feature (contributing guide)

Describe the user story

I have seen pnpm complain about my projects not being set up for pnpm when wanting to use it in a project that had previously been set up to be used w/ npm. I am unsure what heuristics y'all currently use, but why not perhaps also use Corepack's "packageManager" field defined in a project's package.json to more accurately make this determination? It would likely more accurately reflect the intent of the project's developer(s)/maintainer(s).

Describe the solution you'd like

Just consider it for now (is experimental).

Describe the drawbacks of your solution

Can only specify a single version (version ranges are not supported).

Describe alternatives you've considered

This issue is just to put it on your radar, so who knows what might come of it.

[zkochan]

If it only will print a warning or throw an error, then it is not hard to implement. Here is the place where the root package.json file is read: https://github.com/pnpm/pnpm/blob/e19356c32fb87e1555d842cd0fdec7382ce74a91/packages/config/src/index.ts#L527-L530