feat(developer): package file format should include Options.WelcomeFile 🎺

[mcdurdin]

Currently we assume the presence of welcome.htm by name in packages. In the future, this should be a formal reference in the Options object of kps/kmp.json. See also #9477 for how this allows for other file formats such as Markdown.

The Keyman apps would look first at the WelcomeFile property of kmp.json, and if that is missing, fallback to the old welcome.htm fixed filename.

Consider also localization, which applies to any of these metadata files.