fix: Add shuttles into Providence/Stoughton timetable

[joshlarson]

Scope

Asana Ticket: 🏖️⚽️🚄 Update Providence/Stoughton timetable to show match day shuttles

Implementation

This fully just copy-pastes the implementation from #3188, which did the same thing for Franklin/Foxboro shuttles.

The one difference is that the Providence line pulls in its associated shuttle route no matter what, and doesn't hard-code the dates - this still works because there are no shuttles on non-match days, which means that we simply add in an [empty list] on those days, which changes nothing. The Franklin line still needs to use special-cased dates because it has to do some extra work to remove Canton Junction on non-shuttle days, but Providence doesn't have that concern.

Screenshots

*Note: Screenshots are a bit uninspiring right now, because the non-shuttle schedules haven't been updated yet - will add better screenshots once the data's there.*

How to test

Look at schedules for match days and notice that the shuttle trips have been added in.

Look at schedules for non-match days, and notice that there are no shuttles.