Add CRS support to tiles and tile grids

[asinghvi17]

Summary

This PR adds explicit CRS (Coordinate Reference System) support to tiles and tile grids, making it clear which coordinate system a tile is using.

Changes

• Created abstract types AbstractTile and AbstractTileGrid for interface definitions
• Added CRS field to Tile and TileGrid concrete types
• Implemented interface functions:
  • crs(tile) - Get the CRS of a tile/grid
  • x(tile), y(tile), z(tile) - Access tile indices
  • zoom(tile) - Alias for z(tile)
  • bounds(tile) - Get bounds in native CRS
  • bounds(tile, target_crs) - Get bounds projected to target CRS
• Maintained backward compatibility (tiles default to WebMercator when CRS not specified)
• Added comprehensive test suite for new functionality

Test plan

• [x] All existing tests pass
• [x] New tests added for CRS interface (test/test_crs_interface.jl)
• [x] Type stability tests included
• [x] Backward compatibility verified

🤖 Generated with Claude Code

[asinghvi17]

does this approach seem fine @rafaqz @visr? Web merc or wgs84 in this package are singletons so the struct should have the same size and no mutability. This was ai generated so I didn't have it try anything fancy.