Separate I18N calls for immutable and for pluraliseable strings. Fixes #3687

[mgavioli]

Fixes #3687

This PR:

• refactors get_single_section() and get_by_section() — grouped into get() group — by removing the number parameter (previously optional)
• adds get_single_section_w_plural() and get_by_section_w_plural() — grouped into get_n() — by adding a mandatory quantity: int parameter (analogous to the previous number parameter but no longer optional).

Tests in tests/core/text/i18n/test_core_text_i18n.odin have also been updated.

The rationale of this change has been analysed in the #3687 issue and previously in the #3501 discussion.

[Kelimion]

Looks good in principle, but you've inadvertently introduced a lot of typos. I'll do a pass over it.

[mgavioli]

Thanks for merging.

I am sorry for the typos: there was really a lot of them!

VSCode, which I use to work in Odin, does not seem to have a spell checker (and English is not my native language...); I hope it wasn't too long and too boring! In the future, I'll run a spell checking step...