Update Polish translation (`pl_pl.json`)

[lumiscosity]

This PR adds a few new strings which were previously missing, while tackling a few general nitpicks about the translation:

• Shader packs are now consistently referred to as "paczki shaderów" instead of "pakiety Shaderów" to match general Polish translation rules (see "paczki zasobów" for resource packs, for example).
• Similarly, the on/off strings have been changed to a shortened form ("Wł."/"Wył.") to match vanilla.
• A few other existing lines have been reworded to fix grammatical mistakes, wording awkwardness and general confusion.