Add simplify argument to print.compare.loo

[florence-bockting]

Fixes #366

Description

• Restore simplify on print.compare.loo() so users can print the full comparison table again. Default is simplify=TRUE.
• Background: We dropped the argument simplify in the 2.10 loo_compare redesign. User requests (see Re-add print.compare.loo(simplify=) #366) availability of old full print.

Changes

• Add simplify = TRUE to print.compare.loo()
• Keep p_worse / diagnostic columns under the existing p_worse control
• Document the argument and update Examples
• Snapshot + assertion coverage for full print
• Added NEWS entry for the development version

This PR used AI Assistance for drafting the PR.
All changes have been reviewed by a human and adjusted if needed.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.85%. Comparing base (1a9afd3) to head (4d69192).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #383      +/-   ##
==========================================
+ Coverage   92.70%   92.85%   +0.15%     
==========================================
  Files          31       31              
  Lines        3031     3039       +8     
==========================================
+ Hits         2810     2822      +12     
+ Misses        221      217       -4     

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[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if a70c4a3 is merged into master:

• ✔️loo_function: 1.74s -> 1.75s [-0.43%, +1.54%]
• ✔️loo_matrix: 1.73s -> 1.73s [-1.27%, +0.29%]
  Further explanation regarding interpretation and methodology can be found in the documentation.

[jgabry]

Looks great, thanks. I made a couple of trivial change requests.