feat: report undefined `files` array

[privatenumber]

Is your feature request related to a problem? Please describe.

When adding this action to a non-library repo, often times the BASE and HEAD branches have different files to package. The initial repo will usually fail because the base branch includes all files which is too excessive for pkg-size to handle.

Describe the solution you'd like

• Warning on undefined files array (or when HEAD has one defined and BASE doesn't)
• Warning when detected files count exceeds 50 or 1mb

Describe alternatives you've considered

Additional context