Update weight downloading and caching (#130)

[edgararuiz]

Fixes #130

• Removed the auto-download hook from .onAttach(); attaching brulee now has no side effects. TabICL weights download only via tab_icl_download_weights(), and brulee_tab_icl() errors with instructions if they're missing.

• TabICL and Chronos weights now cache under tools::R_user_dir("brulee", "cache") instead of ~/.cache.

• Updated docs, NEWS, and the "not cached" error message.

[topepo]

For TabICL, there is just an error the first time:

Error in `brulee_tab_icl()`:
! No cached regression TabICL checkpoint found in
  /Users/max/Library/Caches/org.R-project.R/R/brulee.
ℹ Download them with `tab_icl_download_weights()`.
Run `rlang::last_trace()` to see where the error occurred.

for Chronos, it just gets them:

> brulee_chronos(ridership ~ Belmont + Clark_Lake, data = modeldata::Chicago)
✔ Downloading <https://huggingface.co/amazon/chronos-2/resolve/0f8a440441931157957e2b…
✔ Downloading <https://huggingface.co/amazon/chronos-2/resolve/0f8a440441931157957e2b…
Chronos-2 Pretrained Forecasting Model

Can you update this PR to,

• if interactive, prompt the user if they want to download them.
• if not interactive, throw an error like what TabICL currently sends