feat(developer): introduce WelcomeFile property to packages

Fixes #9478.

This adds a property WelcomeFile to .kps and kmp.json, which allows us
to move away from the hardcoded welcome.htm filename in the future, and
makes transform from Markdown (#9477) a simpler operation, and just
generally starts the cleanup of the messiness of hard-coded filenames.

The package compiler will fallback to injecting welcome.htm into this
property if (a) welcome.htm is present, and (b) the property does not
already have a value. This doesn't buy us much because we still need to
support welcome.htm for existing legacy packages, but does mean that
our .kmp package metadata will be more consistent for packages compiled
with 17.0+ compilers.