✨ Add "maskify-card-number" challenge

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