feat(map-skip-undefined): add mapSkipUndefined + filterUndefined operator

[dmorosinotto]

This PR substitute #101 to add mapSkipUndefined and filterUndefined instead of old mapFilter proposed in #34
I also added simplified tests using toArray() as suggested by @tomalaforge + docs for both operators.
Hope this can be accepted.

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[dmorosinotto]

@nartc I don't get why CI is failing, the error is on code outside my commit, and before redoing all the work I synced my main branch with yours and then created the new branch to easily merge (fast-forward) my new PR at the end...!?

[eneajaho]

@nartc lets merge this first and then #115