feat: support reading config from package.json

[lukekarrys]

npm@11 has deprecated setting arbitrary config values (npm/cli#8153), which was the previously recommended place for users to set node-gyp related config. The new recommendation from npm is to use the config object in package.json for this (npm/cli#8153 (comment)). I think it makes sense to follow npm's recommendation here.

When a user sets a value in package.json#config it will be set in the environment by npm with the prefix npm_package_config_<KEY>.

I think it makes sense to allow prescribe that they keys be prefixed with node_gyp_. So as an example:

package.json

{
  "config": {
    "node_gyp_devdir": "/tmp/.gyp"
  }
}

Then in node-gyp we can read the env var npm_package_config_node_gyp_devdir. If a user wants to set a global value, they can set that env var on their system.

Fixes #3156

[lukekarrys]

TODO:

• add tests

[lukekarrys]

This PR should have been linked to fixing #3163 and #3192 (not #3156). Not sure how to update this in the changelog now that this has been merged though.