Tests for rewrite hellper

[TorkelE]

Also makes some updates to the functions and adds docstrings

[TorkelE]

With some exceptions ("LuxCore extensions Test" and "Groebner extension Test", which should be unrelated to this), tests passes locally. I think CI can be activated for the PR now (@shashi).

[ChrisRackauckas]

We should probably move ModelingToolkit.fixedpoint_sub here as well?

[ChrisRackauckas]

Missing a docs page.

@YingboMa should review the implementation.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 77.77778% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 76.25%. Comparing base (79c4e92) to head (a5de10a).
Report is 106 commits behind head on master.

Files	Patch %	Lines
src/rewrite-helpers.jl	77.77%	10 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1026      +/-   ##
==========================================
- Coverage   77.07%   76.25%   -0.82%     
==========================================
  Files          32       34       +2     
  Lines        3533     3601      +68     
==========================================
+ Hits         2723     2746      +23     
- Misses        810      855      +45     

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[TorkelE]

I have added entries in the docs now. Symbolics.occursin and Symbolics.filterchildren did not have very natural places, so I added these with Symbolics.replace.

[YingboMa]

LGTM