JS: recognize reading/wrinting calls to fstream

[erik-krogh]

Recognizes code like the below as a writing file-system call (and likewise for reading calls):

var Writer = require('fstream').Writer;
Writer({path: somePath});

This recognizes the sink in CVE-2018-1002203.

[esbena]

LGTM. I wonder how many other missing extends FileSystemWriteAccess we are missing in general. I would have expected extends FileSystemAccess to be sufficient.