Better support for ambiguous lemmatization

This release aligns the behavior of the spaCy pipeline component with standalone Lemmy to support ambiguous lemmatization.

Previously, the pipeline component would return None when more than one possible lemma was returned by Lemmy. The pipeline component now always returns a list of lemmas. For unambiguous words, the list will contain only one lemma but in case of ambiguity, it can contain multiple lemmas. Consequently, the spaCy extension attribute has now been renamed lemmas(plural).