Create percentile.md

https://www.easycalculation.com/statistics/percentile-rank.php

Feel free to try and one-linerify it if possible, lol

snippets/percentile.md

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+### Percentile
+
+Calculate how many numbers are below the value and how many are the same value and
+apply the percentile formula.
+
+```js
+const percentile = (arr, val) => {
+  let below = 0, same = 0;
+  
+  for (const number of arr) {
+    if (number < val) below++;
+    if (number === val) same++;
+  }
+  
+  return 100 * (below + (0.5 * same)) / arr.length;
+};
+// percentile([1,2,3,4,5,6,7,8,9,10], 6) -> 55
+ ```