fix(core): set npm_package_json environment variable when running scripts

[uhop]

What's the problem this PR addresses?

Implements $npm_package_json. Closes #2436

How did you fix it?

Updated scriptUtils.makeScriptEnv to populate the npm_package_json environment variable.

The location of every package root, where package.json resides, is already calculated. I added a way to expose it as $npm_package_json:
* makeScriptEnv() of scriptUtils.ts gets an optional parameter packageLocation.
* If it is supplied, it is joined using ppath.join() with package.json and exposed as $npm_package_json.
* packageLocation comes from initializePackageEnvironment() (the same file), where it finds it using linker.findPackageLocation().

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.

[Fuzzyma]

@merceyz @uhop how far are you with getting this merged? I don't want to let it slide under the rug again where it hides forever :D

[uhop]

@Fuzzyma I am waiting for instructions (see my suggestions and concerns above) from @merceyz or @arcanis or whoever has the authority to accept PRs. If they select an implementation plan acceptable to them, I’ll do it in a jiffy.

[merceyz]

I've updated the implementation to get the packageLocation using the linkers to make sure the value is consistently set when a locator and project is provided.

[uhop]

@merceyz @arcanis — thank you, guys! Many of my users wanted this compatibility feature because they are yarn fans.

If I can help you with merging it in any way — just ask.

[arcanis]

All good - thanks @uhop ! 🌟