Adds a new CI check that all code files have a copyright+license block (and that it's one we are expecting to see).
Fixes the ~350 files (!) that did not have them. This includes all of the files in the .../example/ directories, following the example of flutter/flutter. (This does mean some manual intervention will be needed when generating new example directories in the future, but it's one-time per example.)
Also standardized some variants that used different line breaks than most of the rest of the repo (likely added since I standardized them all a while ago, but didn't add a check for at the time to enforce going forward), to simplify the checks.
Fixesflutter/flutter#77114
