[florence-bockting]

Description

This PR adds a class to each loo warning, following up on our corresponding work in the posteriorpackage: stan-dev/posterior#470.

• Every warning has the class loo_warning and a class for its cause.
• Diagnostic warnings have the class loo_warning_diagnostic.
• The warning text is unchanged and, similar to posterior, a list of warning classes and messages is provided in R/loo-package.R
• Adds rlang to Imports.
• Deletes the unused throw_psis_r_eff_warning() and throw_loo_r_eff_warning() (this was dead code)

This PR is related to Issue #353 (but does not fully solve it).
The PR was assisted by an LLM.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.27273% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.92%. Comparing base (d18de1f) to head (4b90398).

Files with missing lines	Patch %	Lines
R/loo_compare.psis_loo_ss_list.R	35.71%	9 Missing ⚠️
R/loo_subsample.R	0.00%	7 Missing ⚠️
R/psis_approximate_posterior.R	50.00%	2 Missing ⚠️
R/psislw.R	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #417      +/-   ##
==========================================
+ Coverage   92.65%   92.92%   +0.26%     
==========================================
  Files          31       31              
  Lines        3240     3249       +9     
==========================================
+ Hits         3002     3019      +17     
+ Misses        238      230       -8     

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[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 4b90398 is merged into master:

Status	Name	Current	Incoming	Interval
✔️	loo_matrix	1.44s	1.44s	[-0.54%, +0.45%]
✔️	loo_function	1.31s	1.3s	[-0.56%, +0.16%]

Further explanation regarding interpretation and methodology can be found in the documentation.

[jgabry]

Looks good. I made a few suggestions, but pretty small.