Add a badge for parse rate

[alex-pinkus]

This uses the Bring Your Own Badge github action, running a script to
count the number of swift files and generate a percent based on the
count of files in known_failures.

This script only runs once the top-repos job has completed,
successfully, on a code push, so all it needs to do is compare
known_failures against the number of Swift files in the repos.

[alex-pinkus]

I committed this with the if: ${{ github.event_name == 'push' }} clause commented out to validate that it ran successfully. Now that it's done that, gonna uncomment that and amend so that this doesn't run on future PRs.