✨ Add "maskify-card-number" challenge

challenges/maskify-card-number/README.md

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+# maskify-card-number
+
+Created by [@Divlo](https://github.com/Divlo) at 5 July 2020.
+
+## Instructions :
+
+Usually when you buy something, you're asked whether your credit card number, phone number or answer to your most secret question is still correct. However, since someone could look over your shoulder, you don't want that shown on your screen. Instead, we mask it.
+
+Your task is to write a function, which changes all but the last four characters into `'#'`.
+
+## Examples :
+
+See the `input-output.json` file for examples of input/output.

challenges/maskify-card-number/input-output.json

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+[
+  {
+    "input": ["4556364607935616"],
+    "output": "############5616"
+  },
+  {
+    "input": ["1"],
+    "output": "1"
+  },
+  {
+    "input": ["11111"],
+    "output": "#1111"
+  }
+]

challenges/maskify-card-number/solutions/typescript-maskify/README.md

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+# typescript-maskify - maskify-card-number
+
+Programming language : TypeScript
+Created by [@Divlo](https://github.com/Divlo) at 5 July 2020.

challenges/maskify-card-number/solutions/typescript-maskify/solution.ts

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+function solution (string: string) {
+  return string
+    .split('')
+    .map((character, index) => {
+      if (string.length - 4 > index) return '#'
+      return character
+    })
+    .join('')
+}
+
+export default solution