Fix the issue about ESLint and TypeScript.

Author: yoichiro

functions/.eslintrc.js

@@ -17,6 +17,7 @@ module.exports = {
   parserOptions: {
     project: ['tsconfig.json'],
     sourceType: 'module',
+    tsconfigRootDir: __dirname,
   },
   ignorePatterns: [
     '/lib/**/*', // Ignore built files.