chore(null): switch strictNullChecks config from opt-in to opt-out for new files

[dbjorge]

Details

This PR updates our strictNullChecks config and tooling to include all non-test .ts and .tsx files in the strict null checks set by default, only excluding those files explicitly marked as having known issues in tsconfig.strictNullChecks.json. This:

• Simplifies the strictNullChecks config file a lot (it reduces the size by half and is now a flat list of files instead of a mix of files and glob patterns)
• Saves a step of having to manually run yarn null:autoadd for most changes, since new files are now implicitly added
• Removes the corresponding item from our PR checklist (I left the automated PR check in place since it doesn't hurt anything and has a self-descriptive error message if it does fail)

To implement this, I wrote a script to perform the conversion from an include list to an exclude list. I included it for review in this PR, but I don't expect it to need to be run again in the future. I verified that yarn null:progress reports finding the same counts of checked and eligible files before and after the change:

Web strict-null progress

81% complete (1273/1562 non-test files)
Contribute at #2869. Last update: Tue Dec 20 2022

I ensured that the assorted helper scripts (yarn null:find, yarn null:autoadd, yarn null:check) all still work as expected.

Motivation

#2869 and see above

Context

I considered renaming autoadd to something else now that it's arguably "removing an exclusion" rather than "adding an inclusion", but I decided to leave it as it; it's still "adding new files to the strict null checked set" and it felt like more work/confusion than it was worth to try to update all the issues/team working knowledge/etc about it.

Pull request checklist

• Addresses an existing issue: Codebase should use strict null checks #2869
• [x] Ran yarn null:autoadd
• 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

[dbjorge]

Without this change, yarn null:autoadd would try to write out the include: property on multiple lines and conflict with prettier wanting it on one line

[madalynrose]

This is awesome!!