feat: A different approach to warning users of fork() issues with Polars

[itamarst]

Just print a warning when fork() is used at the same time as Polars, which is what jax does. Users aren't as likely to see warnings, but it's something, and it's less intrusive hopefully than the previous attempt that got reverted.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.97%. Comparing base (20ead46) to head (a65272f).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/__init__.py	66.66%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19197      +/-   ##
==========================================
- Coverage   80.03%   79.97%   -0.06%     
==========================================
  Files        1528     1529       +1     
  Lines      209657   209822     +165     
  Branches     2416     2417       +1     
==========================================
+ Hits       167791   167809      +18     
- Misses      41317    41463     +146     
- Partials      549      550       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.