chore(null): add yarn null:progress

[dbjorge]

Description of changes

This implements a new script that generates a progress message for use with our standup dashboard. yarn null:progress emits stdout like this:

## Web strict-null progress

**29%** complete (**387**/1328 non-test files)

*Contribute at [#2869](https://github.com/microsoft/accessibility-insights-web/issues/2869). Last update: Mon Jul 06 2020*

...which renders in the dashboard widget like this:

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