Prepare for modularised quanteda v3

This PR:

calls quanteda.textstats::textstat_entropy() instead of using this from quanteda v2.x
uses more explicit namespacing, so importFrom instead of import for quanteda calls
fixes a few typos (although I did not add spelling checks as part of the testthat.R, will leave this decision to you)

So it will work with both the current quanteda v2.1.2 as well as the forthcoming modularised v3 release.

I did not update NEWS or DESCRIPTION version.

koheiw / newsmap