Add support for loading local XYZ tiles

[rudokemper]

Goal

This PR makes it possible for mbgl-tile-renderer to load local XYZ tiles in addition to MBTiles and GeojSON sources.

What I changed

• Added a fixture with sample raster XYZ tiles and style defining these tiles as a source.
• Added a function getLocalXYZTile to handle the request for XYZ tiles (raster or vector).
• Added a var isXYZDirURL to validate if an incoming request URL is in XYZ format (on the basis of containing /number/number/number), and if so, call the getLocalXYZTile function.