helper: add fs

[hinto-janai]

Adds cuprate_helper::fs for Cuprate-wide directories and files.

A OnceLock & fn() -> &'static Path combination is created for major directories. For now this PR adds:

• cuprate_cache_dir()
• cuprate_config_dir()
• cuprate_data_dir()

with the standards mentioned in #46 using dirs.

[hinto-janai]

Ready for review.

I would be for defining all PATHs used by Cuprate in this module, even if it's only used in 1 place.

• We get to re-use the impl_dir_oncelock_and_fn macro
• All definitions, tests, and documentation would be uniform and in 1 file instead of spread out everywhere

For example, cuprate_database_dir() could be defined here.