Support different types of creations in one repo

[Bergbok]

📝 Provide a description of the new feature

Currently marketplace doesn't allow you to have extensions and CustomApps in the same repo.

It incorrectly displays my extension under the Apps tab.

I've verified that the problem lies with Marketplace, not my manifest.

I suggest that it infers which type each manifest entry is:
if main ends with .js = extension
no main field = customapp
has usercss/schemes fields = theme
has code field = snippet

Wasn't sure whether to classify this as a bug report or feature request but opted for the latter since it seems the current behavior is by design.

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