fix(developer): kmc needs to support .js-only packages

Fixes #9111.

We have existing packages which have only .js in them, for touch-only
keyboards (mostly legacy but still...), so we need to support the freaky
Javascript regex search which we did in the past for extracting
metadata. While this is not necessarily going to work with hand-crafted
Javascript keyboards, it should work with all kmc- and kmcomp-generated
keyboards, so it will suffice to support these legacy packages.

In the future, we will be giving a hint when a package includes a .js
but not a .kmx, gradually upgrading this to a warning and finally an
error as we attempt to phase out .js-based keyboards in preference for
.kmx keyboards.