Overhaul how fonts are handled in theme_thekids() & thekids_table()

Perhaps it is time to fundamentally change our workflows and fully embrace a `systemfonts` type workflow (as suggested by NT). This package is fully supported by tidyverse.

Helpful blog here: https://www.tidyverse.org/blog/2025/05/fonts-in-r/
+ See section "A systemfonts based workflow"
+ Could `ragg` be the answer?
