chore(strict-null): add strict null check migration tools to repo

[dbjorge]

Description of changes

This integrates the strict null checking tools that I migrated from VS Code's as an experiment in https://github.com/dbjorge/aiweb-strict-null-check-migration-tools into the /tools/ folder of our repo, to make it easier for others to use them.

As part of the migration, I eliminated the separate package.json machinery for the tools and had them just re-use the same node_modules as the main project (in particular, re-using the correct typescript version). I also added entry point run-scripts for the tools to our root-level package.json, so the source for the tools should be an implementation detail for most purposes of actually running null check migration work; usage is just yarn null:find and yarn null:autoadd from the root, to match the previously-added yarn null:check.

I've updated #2869 with instructions for how to use these scripts to contribute to the strict null check migration work.

Pull request checklist

• Addresses an existing issue: tools to help address Codebase should use strict null checks #2869
• Ran yarn fastpass
• [n/a] Added/updated relevant unit test(s) (and ran yarn test)
• [n/a] Verified code coverage for the changes made. Check coverage report at: <rootDir>/test-results/unit/coverage
• PR title AND final merge commit title both start with a semantic tag (fix:, chore:, feat(feature-name):, refactor:). See CONTRIBUTING.md.
• [n/a] (UI changes only) Added screenshots/GIFs to description above
• [n/a] (UI changes only) Verified usability with NVDA/JAWS

[karanbirsingh]

one question comment