darts: Add new exercise

exercises/darts/canonical-data.json

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+{
+  "exercise": "darts",
+  "version": "1.0.0",
+  "cases": [
+    {
+      "description": "Return the correct amount earned by a dart landing in a given point in the target problem.",
+      "cases": [
+        {
+         "property": "result",
+         "description": "A dart lands outside the target",
+         "input": {
+           "x": 15.3,
+           "y": 13.2
+         },
+         "expected": 0
+        },
+        {
+         "property": "result",
+         "description": "A dart lands just in the border of the target",
+         "input": {
+           "x": 10,
+           "y": 0
+         },
+         "expected": 1
+        },
+        {
+         "property": "result",
+         "description": "Input is not a number",
+         "input": {
+           "x": "WRONG",
+           "y": 10
+         },
+         "expected": null
+        },
+        {
+         "property": "result",
+         "description": "A dart lands in the middle circle",
+         "input": {
+           "x": 3,
+           "y": 3.7
+         },
+         "expected": 5
+        },
+        {
+         "property": "result",
+         "description": "A dart lands right in the border between outside and middle circles",
+         "input": {
+           "x": 0,
+           "y": 5
+         },
+         "expected": 5
+        },
+        {
+         "property": "result",
+         "description": "A dart arrives in the inner circle",
+         "input": {
+           "x": 0,
+           "y": 0
+         },
+         "expected": 10
+        }
+     ]
+    }
+  ]
+}

exercises/darts/description.md

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+Write a function that returns the earned points in a Darts game.
+
+[Darts](https://en.wikipedia.org/wiki/Darts) is a game where players
+throw darts to a [target](https://en.wikipedia.org/wiki/Darts#/media/File:Darts_in_a_dartboard.jpg).
+
+In our particular instance of the game, the target rewards with 4 different amounts of points, depending on where the dart lands:
+
+* If the dart lands outside the target, player earns no points (0 points).
+* If the dart lands in the outer circle of the target, player earns 1 point.
+* If the dart lands in the middle circle of the target, player earns 5 points.
+* If the dart lands in the inner circle of the target, player earns 10 points.
+
+The outer circle has a radius of 10 units (This is equivalent to the total radius for the entire target), the middle circle a radius of 5 units, and the inner circle a radius of 1. Of course, they are all centered to the same point (That is, the circles are [concentrics](http://mathworld.wolfram.com/ConcentricCircles.html)).
+
+Write a function that given a point in the target (defined by its `real` cartesian coordinates `x` and `y`), returns the correct amount earned by a dart landing in that point.

exercises/darts/metadata.yml

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+---
+blurb: "Write a function that returns the earned points in a Darts game"
+source: "Inspired by an excersie created by a professor Della Paolera in Argentina"