Unify AMT "kiddie tax" child definition with EITC one

[IRC 1(g)(2)(A)](https://www.law.cornell.edu/uscode/text/26/1#g_2_A) and the EITC child definition both point to Section 152. We define those characteristics as `is_eitc_qualifying_child`, e.g. under 18 or under 24 and student. We should unify the logic into a single variable and set of parameters, and rename it to reflect its broader scope than just EITC.