Add references for exemption phase-out threshold inflation adjustments

Low priority since it's not in effect, but these numbers lack citations except the first, which the legal code defines

https://github.com/PolicyEngine/policyengine-us/blob/master/policyengine_us/parameters/gov/irs/income/exemption/phase_out/start.yaml

Open policyengine_us/parameters/gov/irs/income/exemption/phase_out/start.yaml and compare the uncited inflation-adjustment numbers with the first value, whose legal basis is already defined in the code. Research authoritative references for each remaining number and add citations in the file's existing style; the work is done when every adjustment has a source.

Written by the indexing model from the issue text.