Add function to retrieve wordnet synonyms

[ekaf]

Users struggle to find the synonyms of a word (#2976) because the wordnet.py module still does not provide a convenient way to retrieve synonyms, although it claims to:
"Using synsets, helps find conceptual relationships between words such as hypernyms, hyponyms, synonyms, antonyms etc." (sic).

To remedy this deficiency, the present PR adds a function that returns a nested list with the synonyms of the different senses of a word in the given language. Collapsing the different senses into one big list is not recommendable, since the different senses are not mutual synonyms.

from nltk.corpus import wordnet as wn
print(wn.synonyms('car'))

[['automobile', 'machine', 'auto', 'motorcar'], ['railcar', 'railway_car', 'railroad_car'], ['gondola'], ['elevator_car'], ['cable_car']]

print(wn.synonyms('coche', lang='spa'))

[['vehículo', 'auto', 'carro', 'automóvil', 'máquina', 'turismo'], ['vagón', 'automotor'], ['vagón', 'vagón_de_pasajeros']]

[stevenbird]

Thanks @ekaf. Would you mind adding an example to https://github.com/nltk/nltk/blob/develop/nltk/test/wordnet.doctest please? To appear here: https://www.nltk.org/howto/wordnet.html

[stevenbird]

Thanks @ekaf!