add the missing operator `append`

[Joseph Heck]

In the series of Sequence based operators, it seems append has been missed.

https://developer.apple.com/documentation/combine/publisher/3204685-append

(Note: it doesn't work as it's currently documented in Apple's code - append will publish results prior to a completion... so it might be a good idea to double check the tests for other Sequence elements around this issue as well)

[Joseph Heck]

A bit of back and forth on StackOverflow related to this topic: https://stackoverflow.com/questions/60579822/timing-of-values-in-combine-frameworks-append-operator