Add intermediate variables for labor supply responses

For example I'd suggest we create new variables `is_primary_earner_in_tax_unit` and `earnings_decile` from these:
https://github.com/PolicyEngine/policyengine-us/blob/55178d8e923f289b2c52b32187554585a3484e1f/policyengine_us/variables/gov/simulation/labor_supply_response/labor_supply_response.py#L142-L148

We can also modularize the hours worked and earnings effects by creating variables like `relative_substitution_effect` and `relative_income_effect`, then multiplying these by baseline hours and earnings, respectively. We're currently duplicating code between the hours and earnings responses.