feat: support array variables in linear_expansion

[AayushSabharwal]

This is required for SciML/ModelingToolkit.jl#3282. It prevents issues with structural_simplify handling array equations horribly.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 27.27273% with 16 lines in your changes missing coverage. Please review.

Project coverage is 6.51%. Comparing base (6202fa4) to head (d294b0e).
Report is 55 commits behind head on master.

Files with missing lines	Patch %	Lines
src/linear_algebra.jl	27.27%	16 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff            @@
##           master   #1411      +/-   ##
=========================================
+ Coverage    3.95%   6.51%   +2.55%     
=========================================
  Files          51      51              
  Lines        4824    4864      +40     
=========================================
+ Hits          191     317     +126     
+ Misses       4633    4547      -86     

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