Add linter for sync function calls

Summary:
That's another thing I comment on a lot and is a mostly avoidable source of
bad perf.

Reviewed By: mweststrate

Differential Revision: D30450577

fbshipit-source-id: bb82d8cbd34956fa790243f59cda09ff9c4e7379

desktop/.eslintrc.js

@@ -100,6 +100,7 @@ module.exports = {
     'import/no-unresolved': [2, {commonjs: true, amd: true}],
     'node/no-extraneous-import': [2, {allowModules: builtInModules}],
     'node/no-extraneous-require': [2, {allowModules: builtInModules}],
+    'node/no-sync': [1],
     'flipper/no-relative-imports-across-packages': [2],
     'flipper/no-electron-remote-imports': [1],
     'flipper/no-console-error-without-context': [1],