gyp: add common targets

[legendecas]

Fixes: #1379

As documented at https://gyp.gsrc.io/docs/InputFormatReference.md#processing-order, a dependency's setting is processed after merging the root target_defaults, and variable expansion is allowed in dependency specifiers. This allows addons to include node-addon-api and common configs with a single line of gyp config. For example, when the addon needs c++ exception feature, they can simply declare the dependency on node-addon-api-except:

{
  "targets": [
    {
      "target_name": "my_node_addon",
      "sources": [
        "src/my_node_addon.cc"
      ],
      'dependencies': [
        "<!(node -p \"require('node-addon-api').targets\"):node_addon_api_except"
      ]
    }
  ]
}

No need to copy a paragraph of configs into addons' binding.gyp like documented at https://github.com/nodejs/node-addon-api/blob/9aaf3b1/doc/setup.md#installation-and-usage anymore.

• update the document
• add a test

[gabrielschulhof]

Is it all that harmful to leave this as-is and introduce a new key? If there's even a remote chance that it'll break folks, it may be simplest to not reuse this, but introduce a new key, and document what it does.

[gabrielschulhof]

Couldn't folks simply write the following?

...
'dependencies': [
  "<!(node -p \"require('node-addon-api').include_dir\")/node_addon_api.gyp:node-addon-api-except"
]
...

Then we would need only document that node_addon_api.gyp is the way to add node-addon-api to one's binding.gyp.

[legendecas]

It is unnecessary, but having a single gyp entry point would be good. And using the include_dir works -- nothing is wrong. It is simpler to have a gyp file short name, like

'dependencies': [
  "<!(node -p \"require('node-addon-api').include_dir\")/node_addon_api.gyp:node-addon-api-except"
]
// versus...
'dependencies': [
  "<!(node -p \"require('node-addon-api').gyp\"):node-addon-api-except"
]

Would require('node-addon-api').targets sound good to you? For instance,

'dependencies': [
  "<!(node -p \"require('node-addon-api').targets\"):node-addon-api-except"
]

[legendecas]

I've updated the PR to name the entrypoint as require('node-addon-api').targets.

[mhdawson]

LGTM