fix isFile bug

was previously using `fs.lstatSync().isFile()`. The `isFile()` method doesnt seem to exist so i went with `fs.existsSync()` instead.

lib/node.js

@@ -5,9 +5,9 @@ import fs from "fs"
 export const Node = {
   isFile: function(filename) {
     try {
-            return fs.lstatSync(filename).isFile()
+      return fs.existsSync(filename)
     } catch (err) {
-            console.log(err)
+      console.error(err)
       return false
     }