Add MVT Playground example (#14)

mvt-tools.playground/Contents.swift

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+import Foundation
+import MVTTools
+
+/*
+```console
+# Download from MapLibre a vector tile at zoom 2, with an extent showing Norway to India
+wget https://demotiles.maplibre.org/tiles/2/2/1.pbf
+```
+ */
+//: Set the path to the `.pbf` file in `<playground>/Resources`
+let playgroundFile = "maplibre.org_2_2_1.pbf"
+let fileURL = Bundle.main.url(forResource: playgroundFile, withExtension: nil)!
+let mvtData = try Data(contentsOf: fileURL)
+
+//: Load tile using MVT
+let tile = VectorTile(data: mvtData, x: 2, y: 1, z: 2, indexed: .hilbert)!
+
+//: Inspect the properties of the `VectorTile`
+tile.isIndexed
+tile.projection
+
+print(tile.layerNames.sorted())
+//: ### MapLibre ZXY = 2/2/1 — Layers: ["centroids", "countries", "geolines"]
+//: ![](maplibre.org_2_2_1.png)
+//: ---
+//: ### OpenStreetMap ZXY = 2/2/1
+//: ![](openstreetmap.org_2_2_1.png)

mvt-tools.playground/Resources/get_vector_tile.sh

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+wget https://demotiles.maplibre.org/tiles/2/2/1.pbf

mvt-tools.playground/contents.xcplayground

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+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<playground version='5.0' target-platform='macos' display-mode='rendered' buildActiveScheme='true' importAppTypes='true'>
+    <timeline fileName='timeline.xctimeline'/>
+</playground>