feat: support array variables in linear_expansion

[AayushSabharwal]

Before:

julia> @variables x(t)[1:2] y(t)
julia> @parameters f(::Vector{Real})
julia> @mtkbuild sys = ODESystem([D(x) ~ x .* y, y ~ sum(x)], t)
┌ Warning: Internal error: Variable (x(t))[1] was marked as being in 0 ~ -y(t) + Symbolics._mapreduce(identity, +, x(t), Colon(), (:init => false,)), but was actually zero
└ @ ModelingToolkit.StructuralTransformations ~/Julia/SciML/ModelingToolkit.jl/src/structural_transformation/utils.jl:237
┌ Warning: Internal error: Variable (x(t))[2] was marked as being in 0 ~ -y(t) + Symbolics._mapreduce(identity, +, x(t), Colon(), (:init => false,)), but was actually zero
└ @ ModelingToolkit.StructuralTransformations ~/Julia/SciML/ModelingToolkit.jl/src/structural_transformation/utils.jl:237
Model sys with 2 equations
Unknowns (2):
  (x(t))[1]
  (x(t))[2]
Parameters (0):

After:

julia> @mtkbuild sys = ODESystem([D(x) ~ x .* y, y ~ sum(x)], t)
Model sys with 2 equations
Unknowns (2):
  (x(t))[1]
  (x(t))[2]
Parameters (0):

There are also cases where structural_simplify would create DAEs instead of ODEs with those warnings but I can't remember how to MWE that yet.

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Codecov Report

Attention: Patch coverage is 85.71429% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.00%. Comparing base (e036266) to head (408f5fe).
Report is 26 commits behind head on master.

Files with missing lines	Patch %	Lines
src/linear_algebra.jl	85.71%	3 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1320       +/-   ##
===========================================
+ Coverage    3.80%   79.00%   +75.20%     
===========================================
  Files          47       47               
  Lines        4602     4721      +119     
===========================================
+ Hits          175     3730     +3555     
+ Misses       4427      991     -3436     

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[AayushSabharwal]

This is the wrong solution to the problem