enable esModuleInterop in tsconfig to fix import issue

[tylim88]

as for now esModuleInterop is not enabled, this has caused some issues when importing, for example:

import firestore from 'firebase-admin/firestore'
const st = firestore.FieldValue.serverTimestamp() // will not works, `firestore` is undefined

with @babel/preset-env, it is translated into

"use strict";

var _firestore = _interopRequireDefault(require("firebase-admin/firestore"));

function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { default: obj }; }

var st = _firestore.default.FieldValue.serverTimestamp();

playground

this import will not work because, in the built code, we have something like this

in the built code, the __esModule prop is set to true

the problem: import x from 'y' is equivalent to const x = require('y').default but the default prop is not defined(with __esModule set to true, _interopRequireDefault will not return { default: obj } and _firestore.default will become undefined)

the solution is to simply enable esModuleInterop in ts config

======================================
I did the changes needed for esModuleInterop to work and fix some types along the way:

1. enable esModuleInterop in ts config
2. change import * as x to import x for multiple files
3. fix some type issues in:

• test/integration/auth.spec.ts
• test/integration/firestore.spec.ts

no changes in public api

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[tylim88]

my deduction is incorrect

turn out the reason is that it simply doesn't have a default export

this PR is not meaningful, close