new check: inspect files field

[boneskull]

Motivation: npm gives no warning for stuff in files that doesn't exist

New check that should look at the files field and ensure that if the entry allows a file or glob pattern, the resulting packed package actually matches it. This may be a "deny" pattern, so we can safely ignore those, as we can assume that the package manager won't include such files.

Complicating matters is the existence of an .npmignore (is there a yarn/pnpm equivalent?). Unsure yet what to do here, but the behavior should be researched. Does .npmignore override files or vice-versa? Are they simply merged together?

We may also want to check .npmignore because while it's a deny-first list, it may have negated patterns, just like files.