Add unified stemming interface

This commit refactors ever so slightly the '.Stem' modules to create a
unified interface; now Gargantext.Core.Text.Terms.Mono.Stem exports a
single 'stem' function which can be used with different
`StemmingAlgorithm`s, and now the other algorithms' functions have been
shielded behind internal modules.

This makes possible to see at glance which part of Garg are using which
stemming algorithm, as it wasn't always clearer before.

This also allows us to have a better control over the langugages, as not
all the stemming algorithms supports all the languages that Garg
supports.