Make CONTRIBUTORS.txt optional in license headers

[crleonard]

Summary

#267

Update the shared license header check so source headers can omit the
CONTRIBUTORS.txt line.

Some Swift repositories have removed CONTRIBUTORS.txt, so requiring this line
in every source header leaves stale references behind. The check now continues
to accept existing headers that include the line, but also accepts headers where
the line has been removed.

Details

The checker still validates the full expected header first. If that comparison
fails, it retries after removing any CONTRIBUTORS.txt line from both the
expected header and the file header.

This keeps compatibility for repositories that still include the line while
allowing repositories that have removed CONTRIBUTORS.txt to update their
headers.

Testing

Added tests/check-license-header.sh, which creates a temporary git repo with
two Swift files:

• one header with the CONTRIBUTORS.txt line
• one header without the CONTRIBUTORS.txt line

Both are checked using the real license header script.

Manually verified with:

tests/check-license-header.sh
shellcheck tests/check-license-header.sh .github/workflows/scripts/check-license-header.sh
PROJECT_NAME=Swift.org .github/workflows/scripts/check-license-header.sh