Line Colors / Stop Names in Debug Tiles

Our debug tiles could use some ❤️ . The tiles already contain information about [stop names](https://github.com/mapbox/directions-transit/blob/187941019fdbacf84b9b0fe29ff8fefff3e68686/src/service/tile.cpp#L97-L99) which is not rendered on the map.

In addition, it would be lovely to add the correct line colours to the lines, as long as they are specified (see gtfs documentation [here](https://developers.google.com/transit/gtfs/reference/routes-file)).

In case of not-specified colours, we could probably try and use route types to select colours by category.

In an added effort, visualising wheelchair accessibility somehow, maybe bike transport. If wheelchair access would be visible, this could be a huge bonus and lay groundwork for searching routes based on accessibility.

- [ ] pull in colours from `routes.txt`
- [ ] assign `lines` their correct colour (this should be an annotation feature, mapping lineID->hex code) (maybe compare [trip annotation](https://github.com/mapbox/directions-transit/blob/187941019fdbacf84b9b0fe29ff8fefff3e68686/include/annotation/trip.hpp) for inspiration
- [ ] augment the tile service [here](https://github.com/mapbox/directions-transit/blob/187941019fdbacf84b9b0fe29ff8fefff3e68686/src/service/tile.cpp#L58) to make use of the new annotation feature
- [ ] add front-end support for the line colours (probably around [here](https://github.com/mapbox/directions-transit/blob/187941019fdbacf84b9b0fe29ff8fefff3e68686/frontends/debug/index.html#L51-L65))

Read src/service/tile.cpp and include/annotation/trip.hpp, then inspect frontends/debug/index.html and the GTFS routes.txt fields. Trace how debug tiles currently expose stop names. Done means stop names and specified line colours are represented in tiles and rendered by the debug frontend; route-type fallbacks and accessibility or bike support are optional ideas in the issue.

Written by the indexing model from the issue text.