chore(developer): add --for-publishing to kmc

The new --for-publishing flag allows us to enforce additional
requirements for keyboards and lexical models that are to be published
to the keymanapp repositories. This flag will always be switched on in
the repository builds. This flag overrides the 'skipMetadata' option
which is a project-level option.

There is a bit of a delicate balance of requirements here:

1. We want to be able to automatically verify keyboards and package
   licenses if they are in the repository.
2. We do not want to force license checks on privately built keyboards
   and models.
3. We want to provide pathways for users to check locally before
   submitting to the repository.
4. If possible, we want to be able to build the .keyboard_info and
   .model_info files locally, but this should be up to the user for
   local builds.

For now, the only additional check that this flag provides is to verify
that the license is MIT, which was an unchecked requirement for the
repositories in the past.

Future checks can be added for file layout, additional required
metadata files.

Anticipate adding this as a tool to Keyman Developer IDE -- a
'pre-publish' check -- in the future.