Have eslint ignore undefined variable "exports"

Modules defined using goog.module declare their exported functions
by assigning them to properties on an object which is the value of
the variable "exports" (e.g., "exports.MyClass = class { ... };").

Normally eslint would complain about this variable being undefined,
but this commit suppresses these errors.

.eslintrc.json

@@ -79,7 +79,8 @@
     },
     "globals": {
         "Blockly": true,
-        "goog": true
+        "goog": true,
+        "exports": true
     },
     "extends": [
         "eslint:recommended"