Create reduce.md exercise

docs/Exercise/JavaScript_Basics/reduce.md

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+# Reduce exercises
+
+The reduce method allows you to "reduce" the contents of an Array into one value.
+
+## 1. Sum of prices
+Write a function that sums up the prices in a shopping cart.
+
+```js
+const items = [
+  { name: 'Flour', price: 1 },
+  { name: 'Tomatos', price: 4 },
+  { name: 'Cucumbers', price: 2 },
+  { name: 'Cheese', price: 7 },
+  { name: 'Wine', price: 14 },
+];
+
+const expectedResult = 28;
+```
+
+## 2. Grades by course
+Given a list of grades with their course, return an object where the key is the name of the course and the value is a list of the corresponding grades.
+
+```js
+const grades = [
+  { course: 'Algebra', test: 1, grade: 'A'},
+  { course: 'Algebra', test: 2, grade: 'C'},
+  { course: 'Algebra', test: 3, grade: 'B'},
+  { course: 'Algorithms & Datastructures', test: 1, grade: 'D'},
+  { course: 'Algorithms & Datastructures', test: 2, grade: 'C'},
+  { course: 'Algorithms & Datastructures', test: 3, grade: 'C+'},
+  { course: 'English', test: 1, grade: 'B'},
+  { course: 'English', test: 2, grade: 'C'},
+  { course: 'English', test: 3, grade: 'B'},
+];
+
+const expectedResult = {
+  'Algebra': [
+    { course: 'Algebra', test: 1, grade: 'A'},
+    { course: 'Algebra', test: 2, grade: 'C'},
+    { course: 'Algebra', test: 3, grade: 'B'},
+  ],
+  'Algorithms & Datastructures': [
+    { course: 'Algorithms & Datastructures', test: 1, grade: 'D'},
+    { course: 'Algorithms & Datastructures', test: 2, grade: 'C'},
+    { course: 'Algorithms & Datastructures', test: 3, grade: 'C+'},
+  ],
+  'English': [
+    { course: 'English', test: 1, grade: 'B'},
+    { course: 'English', test: 2, grade: 'C'},
+    { course: 'English', test: 3, grade: 'B'},
+  ]
+};
+```