feat: add exercise jabs

subjects/jabs/README.md

@@ -0,0 +1,14 @@
+## jabs
+
+
+### Instructions
+
+- Create a function named `abs` that takes a number as an argument and returns its absolute value. You must make your own implementation. You must not use `Math.abs()`
+
+### Expected function
+
+```js
+function abs(n) {
+
+}
+```