chore(developer): verify file types of content files in package

This is a transfer of the functionality in the legacy package compiler;
we could go much further in verifying file types and excluding certain
files, but that's a big design session. For now, just refreshed to
include the set of Keyman for Windows and Keyman Engine for Windows
files which are most likely to be accidentally included.

Adds WARN_RedistFileShouldNotBeInPackage and WARN_DocFileDangerous and
corresponding unit tests and constant declarations.