Add countSetBits (#152)

src/algorithms/math/bits/countSetBits.js

@@ -0,0 +1,13 @@
+/**
+ * @param {number} number
+ * @return {number}
+ */
+export default function countSetBits(number) {
+  let count = 0;
+  let num = number; // eslint error https://eslint.org/docs/rules/no-param-reassign
+  while (num) {
+    count += num & 1;
+    num >>= 1;
+  }
+  return count;
+}