- do not use the .d.ts files coming in from gulp-tsb
- remove the generation of .d.ts files
- run before compilation starts
- install file watchers directly

Fixes #61000

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 #60565

Adds a new `tsconfig.strictNullChecks.json` project that does not emit anything and is only used for enabling strict null checks on a subset of the vscode codebase.

Opt `iterator.ts` into strict null checking.

Fix our build scripts to properly handle `extends`

Tree Shaking:
- do not proceed with tree shaking when there are compilation errors
- load .d.ts files in the language service
- adopt TS 3.1.1 in symbol resolution
- use the real tsconfig.json / with "node" resolution

Bundling:
- fix issue where files were being looked for in out-build instead of out-editor-build