chore: add 'files' field to package.json

[nktnet1]

Hello,

In version 3.0.0, the code tab on npm contains many redundant files:

From what I can see, only

  "files": [
    "binding.gyp",
    "dist",
    "scripts",
    "src"
  ],

is necessary to install node-libcurl.

For example, I made a fork of node-libcurl called easy-libcurl@1.0.3 containing the "files" attribute above:

which installs and runs fine. While easy-libcurl only exports the Easy interface, I doubt this will have an effect on which files are necessary for the distributable.

This change will reduce the package size when installed. Hope it helps in some way :).

Thank you again for your work on node-libcurl!