chore: start incremental migration to strictNullChecks

[dbjorge]

Description of changes

This PR, plus the scripts in dbjorge/aiweb-strict-null-check-migration-tools, creates the initial infrastructure for an incremental migration to strictNullChecks: true, following the same strategy that VS Code used.

• Adds a secondary tsconfig file at tsconfig.strictNullChecks.json which is the same as our main/normal tsconfig file except that it enables strictNullChecks and only runs on the already-compliant parts of our code
• Populates the initial files and includes of the strictNullChecks config with the subset of /src/ that is already compliant (by using autoAdd from aiweb-strict-null-check-migration-tools
• Adds a new yarn null:check command to compile against the new strictNullChecks config (note, an especially useful option during cleanup work is yarn null:check --watch)
• Adds yarn null:check into yarn fastpass
• Adds yarn null:check into PR/CI build

The ongoing effort to incrementally bring the codebase to null safety would amount to iteratively using aiweb-strict-null-check-migration-tools to find the next candidate files, fixing up null safety issues there, and using that repo's autoAdd tool to bring in any transitively-fixed cases.

Pull request checklist

• Addresses an existing issue: 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