chore: add issue templates

[e-lo]

As a contributor/reviewer of GTFS, I'd like to understand what is expected for the following types of issues when submitting them, AND would appreciate a common structure when reviewing/commenting on them:

• Structural/major changes to GTFS (i.e. Fares v2)
• General suggested updates to the spec.
• Updates in descriptions/language which clarify the existing intent.

[lauramatson]

This would be very helpful! I might add a category for raising issues / gaps / questions that someone doesn't have a proposed solution for yet. Maybe that's encompassed by the second bullet point or maybe it's a precursor to it?

[e-lo]

This is being worked on here: MobilityData#64