feat: export FileSystem and FtpSrv

Non breaking, as it still exports FtpSrv by default

ftp-srv.js

@@ -0,0 +1,6 @@
+const FtpSrv = require('./src');
+const FileSystem = require('./src/fs');
+
+module.exports = FtpSrv;
+module.exports.FtpSrv = FtpSrv;
+module.exports.FileSystem = FileSystem;

package.json

@@ -11,7 +11,7 @@
     "server"
   ],
   "license": "MIT",
-  "main": "src/index.js",
+  "main": "ftp-srv.js",
   "files": [
     "src"
   ],