feat(solution): add typescript-caesar-cipher

challenges/caesar-cipher/solutions/typescript-caesar-cipher/README.md

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+# typescript-caesar-cipher - caesar-cipher
+
+Programming language : TypeScript
+
+Created by [@Divlo](https://github.com/Divlo) on 21 October 2020.

challenges/caesar-cipher/solutions/typescript-caesar-cipher/solution.ts

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+const alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'.split('')
+
+interface ShiftedLetter {
+  origin: string
+  shifted: string
+}
+
+function shiftAlphabet (shift: number): ShiftedLetter[] {
+  const result: ShiftedLetter[] = []
+  for (let index = 0; index < alphabet.length; index++) {
+    const letter = alphabet[index]
+    let shiftedIndex = index + shift
+    if (shiftedIndex > alphabet.length - 1) {
+      shiftedIndex = Math.abs(alphabet.length - shiftedIndex)
+    }
+    result.push({
+      origin: letter,
+      shifted: alphabet[shiftedIndex]
+    })
+  }
+  return result
+}
+
+function solution (str: string, shift: number): string {
+  const shiftedAlphabet = shiftAlphabet(shift)
+  let result = ''
+  for (const letter of str) {
+    if (letter === ' ') {
+      result += ' '
+    } else {
+      for (const { origin, shifted } of shiftedAlphabet) {
+        if (letter === shifted) {
+          result += origin
+          break
+        }
+      }
+    }
+  }
+  return result
+}
+
+export default solution