title: Read and Write file using vs code extension
body: <p>i am building an extension to parse json using vs code extension.
  so my need is ,it should be able able to load .json file from a particular folder and iterate through content of the file.
  Then it should allow user to select few keys from it make a new json file out of this and save it in any folder.</p>
  
  <p>But i am not able to find any way to read and write files in "vs code extension".Could someone please help me.</p>