Refactor the `regular_tax_before_credits` variable and add unit tests

[PavelMakarchuk]

It currently includes a number of variables which are not descriptive as well as calculation steps which do not seem transparent

First step would be to compare the calculation to capital_gains_tax (up until the inclusion of income_tax_main_rates) and to identify which steps can be consolidated for clarity

Secondly, unit tests should be added to test the file - it will be not done in #4815

We should also add references and tie the hard coded 0.28 parameter from the dwks41 calculation to the amt bracket parameter which is refactored in #4815

[MaxGhenis]

The last part is #3279

[PavelMakarchuk]

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