feature request: emit @mismatch when subslot filters out invalid elements

[pretzelhammer]

Currently subslot only emits a no-match event if no valid elements are found, however it would be very nice if it also emitted a mismatch event when invalid elements are found. My use-case is I'm using subslot to validate the components a user is passing in through a slot and if they pass an invalid component I would like to log a console warning. Thanks!

[privatenumber]

If using the mixin API, all non-matching vnodes should be in this.$subslots.default.

This might not yield what you want if you have multiple subslots though.

I want to be careful with keeping this as a slot filteration library rather than a validation one, but I will think about this some more.