Mark caesar as archived and move archived exercise to own folder (#336)

archive/archived_caesar/solution/caesar-solution.js

@@ -0,0 +1,26 @@
+const caesar = function (string, shift) {
+  return string
+    .split("")
+    .map((char) => shiftChar(char, shift))
+    .join("");
+};
+
+const codeSet = (code) => (code < 97 ? 65 : 97);
+
+// this function is just a fancy way of doing % so that it works with negative numbers
+// see this link for details:
+// https://stackoverflow.com/questions/4467539/javascript-modulo-gives-a-negative-result-for-negative-numbers
+const mod = (n, m) => ((n % m) + m) % m;
+
+const shiftChar = (char, shift) => {
+  const code = char.charCodeAt();
+
+  if ((code >= 65 && code <= 90) || (code >= 97 && code <= 122)) {
+    return String.fromCharCode(
+      mod(code + shift - codeSet(code), 26) + codeSet(code)
+    );
+  }
+  return char;
+};
+
+module.exports = caesar;