pkg extension for Vcov

[palday]

note: this is not yet correct

• add entry in NEWS.md
• after opening this PR, add a reference and run docs/NEWS-update.jl to update the cross-references.
• I've bumped the version appropriately

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 96.64%. Comparing base (84a61c2) to head (d7d1a05).

Files	Patch %	Lines
ext/MixedModelsVcovExt.jl	0.00%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #768      +/-   ##
==========================================
- Coverage   96.93%   96.64%   -0.29%     
==========================================
  Files          34       35       +1     
  Lines        3356     3366      +10     
==========================================
  Hits         3253     3253              
- Misses        103      113      +10     

Flag	Coverage Δ
current	96.58% <0.00%> (-0.29%)	⬇️
minimum	96.64% <0.00%> (-0.20%)	⬇️
nightly	96.14% <0.00%> (-0.29%)	⬇️

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