Break ModelBuilder into smaller classes

[williambdean]

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Description

Splitting the original ModelBuilder into base class which then supports ModelBuilder and RegressionModelBuilder

Work in progress. Likely very broken at the moment

Related Issue

• [ ] Closes #
• [ ] Related to #

Checklist

• [ ] Checked that the pre-commit linting/style checks pass. Feel free to comment pre-commit.ci autofix to auto-fix.
• [ ] Included tests that prove the fix is effective or that the new feature works
• [ ] Added necessary documentation (docstrings and/or example notebooks) using numpydoc format.
• [ ] If you are a pro: each commit corresponds to a relevant logical change

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📚 Documentation preview 📚: https://pymc-marketing--1467.org.readthedocs.build/en/1467/