chore(scripts): normalize paths and include typeRoots for Windows (#11933)

Short description of what this resolves:

Allows our e2e tests to run on a Windows machine

Changes proposed in this pull request:

Normalize paths using path.join()
Add typeRoots to the root tsconfig file
Change the spawnedCommand based on the OS
Fixes: #11551

scripts/gulp/tasks/polyfill.source.ts

@@ -1,9 +1,10 @@
 import { task } from 'gulp';
 import { writePolyfills } from '../util';
+import { join } from 'path';
 
 
 task('src.polyfill', (done: Function) => {
-  writePolyfills('scripts/polyfill').then(() => {
+  writePolyfills(join('scripts', 'polyfills')).then(() => {
     done();
   }).catch(err => {
     done(err);