Fixes #56429

Part of #60565

Clean up activity bar icons

Fixes #59317

fixes #60593

This reverts commit 0c0d368e.

These checks are now taking a few seconds to run. This will continue to slow down as we null check more files. Moving them to the test scripts as pretest checks instead

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`

Fixes #59379