Centralize processing of monomial terms in LinearRepn compiler

[jsiirola]

Fixes # .

Summary/Motivation:

This is the first step in reworking the core monomial recording logic in the "new" (LinearRepn / QuadraticRepn) compilers. Having a single method for recording monomial terms into an already-created Repn data structure will make it easier to do custom overloads (e.g., in ParameterizedRepn walkers) and to experiment with different Repn internal data formats. It also removes "in-lined" (copy/pasted) code.

The cost of this is an extra function call when processing terms. This has a measurable, albeit small impact on the LP writer:

Changes proposed in this PR:

• centralize all processing of monomial terms (in the LinearBeforeChildDispatcher) in a record_monomial method
• update the base LinearBeforeChildDispatcher _before_var, _before_monomial and _before_linear methods.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.10%. Comparing base (6ffd3cb) to head (2b5b655).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3956   +/-   ##
=======================================
  Coverage   90.10%   90.10%           
=======================================
  Files         908      908           
  Lines      108344   108325   -19     
=======================================
- Hits        97620    97606   -14     
+ Misses      10724    10719    -5     

Flag	Coverage Δ
builders	29.13% <44.11%> (+0.01%)	⬆️
default	86.09% <100.00%> (?)
expensive	35.14% <64.70%> (?)
linux	87.61% <100.00%> (-1.99%)	⬇️
linux_other	87.61% <100.00%> (+<0.01%)	⬆️
oldsolvers	28.07% <88.23%> (+<0.01%)	⬆️
osx	83.05% <100.00%> (-0.01%)	⬇️
win	85.36% <100.00%> (+<0.01%)	⬆️

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