Customize Latexified variable names with function keyword argument

[hersle]

I'm exploring options. Here is an alternative to #1350, using a keyword argument (similar to what @isaacsas suggested). Example usage:

# 1) Default Symbolics-only pure behavior
using Symbolics, Latexify
@variables car₊wheel₊ω
latexify(car₊wheel₊ω) # car.wheel.\omega

# 2) Default in ModelingToolkit?
formatter_mtk(name) = "\\mathtt{$name}"
latexify(car₊wheel₊ω; variable_formatter = formatter_mtk) # \mathtt{car.wheel.\omega}

# 3) Default in another package?
formatter_subsystem(name) = "\\mathrm{$name}"
formatter_variable(name) = length(name) > 1 ? "\\mathrm{$name}" : name
formatter_advanced(name) = (names = split(name, "."); join([formatter_subsystem.(names[begin:end-1]); formatter_variable(names[end])], "."))
latexify(car₊wheel₊ω; variable_formatter = formatter_advanced) # \mathrm{car}.\mathrm{wheel}.\omega

MTK or other packages could dispatch on latexify(sys::SystemType) with an appropriate formatting function.

[hersle]

@latexrecipe is a little finicky with keyword arguments, particularly when they are functions or declared as kwargs.... Hence the small hack. See korsbo/Latexify.jl#320.

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Codecov Report

Attention: Patch coverage is 0% with 40 lines in your changes missing coverage. Please review.

Project coverage is 6.47%. Comparing base (5af597a) to head (a68360a).
Report is 11 commits behind head on master.

Files with missing lines	Patch %	Lines
src/latexify_recipes.jl	0.00%	40 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff            @@
##           master   #1352      +/-   ##
=========================================
+ Coverage    3.98%   6.47%   +2.48%     
=========================================
  Files          50      50              
  Lines        4771    4775       +4     
=========================================
+ Hits          190     309     +119     
+ Misses       4581    4466     -115     

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[hersle]

Forget this, it has the same flaw as #1350.