Add new entries to gazetteers

[dasulit]

Add a handful of new gazetteer entries that reflect features highlighted in style components.

Updated gazetteers:

• worldwide/natural-labels.json
• worldwide/points-of-interest.json
• worldwide/road-network.json
• worldwide/transit.json

New gazetteers:

• worldwide/admin-boundaries.json
• worldwide/buildings.json
• worldwide/land-and-water.json
• worldwide/terrain.json
• worldwide/walking-cycling.json

@ClareTrainor for review.

[dasulit]

Thanks @ClareTrainor for looking closely! Updated based on your suggestions. Gonna merge!