Various improvements

[WasabiThumb]

• Generated missing package-lock.json, this is valuable for deterministic installs
• Normalized contract
  • Promise now always resolves with the output code, this is so that the typing doesn't need to have a special case. Side effects should be non-existent as dependents that are built with the assumption that the API will return a void promise should now quietly discard the output code.
    • Before

      await UglifyPHP.minify("in.php", { output: "out.php" }); // void
      await UglifyPHP.minify("in.php"); // string

    • After

      await UglifyPHP.minify("in.php", { output: "out.php" }); // string
      await UglifyPHP.minify("in.php"); // string

• Added types for compatibility with TypeScript
• Added documentation to types
• Applied lints
  • var -> let, const
  • == -> === (for string to string comparison, should be identical)
• Added method to API that accepts only source code; this means that using statSync to check if the input is a path is not required
• Added filesOnly option