Fix defunct dfm and tokens usages

[kbenoit]

These fixes will ensure that your package does not break when we publish quanteda v4.

Breaking changes:

• dfm.character() is deprecated in v3 and defunct in v4
• stem is no longer an argument for tokens() or dfm()

Note also that dfm() in quanteda v4 is also faster by up to 30% as we re-implemented it in C++.