QBID variables only

[baogorek]

Closes #6068

New QBID Variables are necessary to do the computations we want to do.

The Larger Plan. Complete in this order:

1. THIS ONE - policyengine-us: QBID Variables Only # 6031 - there's no logic in here. Just variables
2. policyengine-us-data: QBID Variables added #256 - We need 1 in before we can pass these tests
3. policyengine-us: QBID formula Logic and Tests #6067 - relies on the first two, but now we're down to 7 files

[MaxGhenis]

while here can we refactor to add some intermediate variables? we generally try to keep each under ~50 lines, these are 100+

[MaxGhenis]

good to merge after these