armstrong-numbers: new exercise

exercises/armstrong-numbers/canonical-data.json

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+{
+  "exercise": "armstrong-numbers",
+  "version": "1.0.0",
+  "cases": [
+    {
+      "description": "Single digit numbers are Armstrong numbers",
+      "property": "isArmstrongNumber",
+      "input": {
+        "number": 5
+      },
+      "expected": true
+    },
+    {
+      "description": "There are no 2 digit Armstrong numbers",
+      "property": "isArmstrongNumber",
+      "input": {
+        "number": 10
+      },
+      "expected": false
+    },
+    {
+      "description": "Three digit number that is an Armstrong number",
+      "property": "isArmstrongNumber",
+      "input": {
+        "number": 153
+      },
+      "expected": true
+    },
+    {
+      "description": "Three digit number that is not an Armstrong number",
+      "property": "isArmstrongNumber",
+      "input": {
+        "number": 100
+      },
+      "expected": false
+    },
+    {
+      "description": "Four digit number that is an Armstrong number",
+      "property": "isArmstrongNumber",
+      "input": {
+        "number": 9474
+      },
+      "expected": true
+    },
+    {
+      "description": "Four digit number that is not an Armstrong number",
+      "property": "isArmstrongNumber",
+      "input": {
+        "number": 9475
+      },
+      "expected": false
+    },
+    {
+      "description": "Seven digit number that is an Armstrong number",
+      "property": "isArmstrongNumber",
+      "input": {
+        "number": 9926315
+      },
+      "expected": true
+    },
+    {
+      "description": "Seven digit number that is not an Armstrong number",
+      "property": "isArmstrongNumber",
+      "input": {
+        "number": 9926314
+      },
+      "expected": false
+    }
+  ]
+}

exercises/armstrong-numbers/description.md

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+An [Armstrong number](https://en.wikipedia.org/wiki/Narcissistic_number) is a number that is the sum of its own digits each raised to the power of the number of digits.
+
+For example:
+
+- 9 is an Armstrong number, because `9 = 9^1 = 9`
+- 10 is *not* an Armstrong number, because `10 != 1^2 + 0^2 = 2`
+- 153 is an Armstrong number, because: `153 = 1^3 + 5^3 + 3^3 = 1 + 125 + 27 = 153`
+- 154 is *not* an Armstrong number, because: `154 != 1^3 + 5^3 + 4^3 = 1 + 125 + 64 = 190`
+
+Write some code to determine whether a number is an Armstrong number.

exercises/armstrong-numbers/metadata.yml

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+---
+blurb: "Determine if a number is an Armstrong number"
+source: "Wikipedia"
+source_url: "https://en.wikipedia.org/wiki/Narcissistic_number"