Parameterize 25% on line 37 of AMT

[MaxGhenis]

We currently hard-code this:

policyengine-us/policyengine_us/variables/gov/irs/tax/federal_income/alternative_minimum_tax.py

Line 78 in c6ecdc2

dwks38 = 0.25 * dwks37

See https://www.irs.gov/pub/irs-pdf/f6251.pdf (now line 37 instead of 38)

I think this is represented in https://www.law.cornell.edu/uscode/text/26/55#b_3_E

The amount determined under the first sentence of paragraph (1)(A) shall not exceed the sum of— [...] 25 percent of the amount of taxable excess in excess of the sum of the amounts on which tax is determined under the preceding subparagraphs of this paragraph.

AMT parameters are in https://github.com/PolicyEngine/policyengine-us/tree/master/policyengine_us/parameters/gov/irs/income/amt

ChatGPT thought capital_gain_excess_tax_rate would be an appropriate name: https://chat.openai.com/share/35162ac1-0ac3-4caa-91b8-4ee34f7879d5

Note Tax-Calculator also hard-codes this, see PSLmodels/Tax-Calculator#2635