chore(null): update yarn null:autoadd to run multiple passes

[dbjorge]

Description of changes

Previously, if yarn null:autoadd added a file to the strictNullChecks config which enabled a previously-ineligible file to be considered for auto-add, you'd have to run yarn null:autoadd again and eat the cost of it spinning up a new tsc watch process and rescanning all the already-known-failures.

With this PR, the tool will do this for you automatically, re-using both the watch process and previously-established knowledge of known failures.

yarn null:progress update with this PR:

Web strict-null progress

42% complete (558/1334 non-test files)

Contribute at #2869. Last update: Fri Sep 11 2020

Pull request checklist

• Addresses an existing issue: part of 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