Mark windows-shell setting as deprecated

[terror]

Marks the windows-shell setting as deprecated (just master, casey/just#3536), surfacing deprecation diagnostics and completion metadata. The stock rename quickfix is skipped since the replacement is a pattern change ([windows] attribute on set shell), not a rename.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.73418% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.12%. Comparing base (eab0c1a) to head (f9497e1).

Files with missing lines	Patch %	Lines
src/deprecation.rs	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #464      +/-   ##
==========================================
+ Coverage   95.10%   95.12%   +0.02%     
==========================================
  Files          60       62       +2     
  Lines       13371    13449      +78     
==========================================
+ Hits        12716    12793      +77     
- Misses        655      656       +1     

Flag	Coverage Δ
unit	95.12% <98.73%> (+0.02%)	⬆️

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[terror]

How should the diagnostic behave when a file contains both the deprecated windows-shell setting and an equivalent [windows] set shell declaration? A test for that mixed configuration could ensure the server reports deprecation without offering a misleading automated edit or producing duplicate/conflicting diagnostics. It may also be worth documenting that migration requires a structural rewrite rather than a simple key rename.

You raise a good point, and I'm still a bit hesitant for this PR. Going to have to give it a bit more thought.