feat: add experimental support for Node.js subpath imports

[fraxken]

Add initial support of Node.js subpath imports (see https://nodejs.org/api/packages.html#subpath-imports).

const foo = require("#dep");

And in package.json:

{
  "imports": {
    "#dep": {
      "node": "kleur"
    }
  }
}

So in the code foo is equal to kleur dependency.