Added uniqueSymmetricDifference and updated symmetricDifference

snippets/uniqueSymmetricDifference.md

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+### uniqueSymmetricDifference
+
+Returns the unique symmetric difference between two arrays.
+
+Use `Array.filter()` and `Array.includes()` on each array to remove values contained in the other, then create a `Set` from the results, removing duplicate values.
+
+```js
+const uniqueSymmetricDifference = (a, b) => [
+  ...new Set([...a.filter(v => !b.includes(v)), ...b.filter(v => !a.includes(v))])
+];
+```
+
+```js
+uniqueSymmetricDifference([1, 2, 3], [1, 2, 4]); // [3, 4]
+uniqueSymmetricDifference([1, 2, 2], [1, 3, 1]); // [2, 3]
+```