t5s3-epaper: Move variant.cpp -> extra_variants/variant.cpp ...again (#10297)

Fixes issues with #includes inherited from `configuration.h` when building for pioarduino.

Aligns t5s3_epaper with other variants like t_deck_pro.

Co-authored-by: Copilot <copilot@github.com>
Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

Author: 3 people