Upgrades our build scripts to target ES2017 since they are run on modern versions of node
This allows us to remove shims for es6 features such as `Object.assign`, and also remove a few extra typings packages
Part of #24698
**bug**
A node_modules folder under build/lib/watch is being included in our project system. This slows down TS quite a lot
**fix**
Remove this and remove the root jsconfig.json in the project