feat: add support for typed function symbolics

[AayushSabharwal]

Draft while the other PRs in SciML/ModelingToolkit.jl#2910 are written, so that the entire thing can be tested together.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 87.87879% with 8 lines in your changes missing coverage. Please review.

Project coverage is 80.81%. Comparing base (8cb25a8) to head (7902377).
Report is 16 commits behind head on master.

Files with missing lines	Patch %	Lines
src/variable.jl	87.87%	8 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1270       +/-   ##
===========================================
+ Coverage   30.06%   80.81%   +50.74%     
===========================================
  Files          47       47               
  Lines        4576     4728      +152     
===========================================
+ Hits         1376     3821     +2445     
+ Misses       3200      907     -2293     

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