Add all eligibility checks needed for phlPreK and paSeniorFoodBox benefits

[prestoncabe]

• [x] add prek checks to eligibility.dmn
• [x] add food box checks to eligibility.dmn
• [x] (extra credit: wire the checks up to their respective benefits)

There are a few checks implemented already (as decision services) in eligibility.dmn, including:

• childAgeThreeOrFourSeptFirst
• someoneAgeSixtyOrOlder

We can follow those examples to add additional checks.

Note that we will likely need to flesh out our inputs to include additional information in order to fully implement the checks needed for these two benefits.