fix: remove problematic process-shared global variables

[BoboTiG]

Definitively fixes #1054, and simplify the thinking process while adding more explicit code.

It seems a lot of code, maybe that's not the best approach, but at least it feels more proper & explicit: I added the missed_templates keyword-argument from top calls until the lowest function. It will work either with a simple list (like when used from --get-word, or --check-word), a shared multiprocessing object (like in --render), and it does not break other code since it is None by default.
No more global variables, and I tested with all multiprocessing start methods (fork, forkserver, and spawn): they all work.

Bonus points:

• --check-words now reports missed templates
• no more side-effects in tests: each run has its own missed_template value (as seen in test_2_render.py clean-up)

[BoboTiG]

Side note: While adapting the code to all locales, I saw that English (EN) is not calling the default last template handler function, and so it has/will never report missed templates.

[BoboTiG]

/cc @lasconic