Part of #60565

Fixes #59317

fixes #60593

This reverts commit 0c0d368e.

Uses scripts from https://github.com/mjbvz/vscode-strict-null-check-migration-tools

Fixes #60524

These properties are not inherited

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`

This reverts commit bab47f0f.

fixes #60444

fixes #60018

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