implement sum helper as strict alias for add helper (#614)

addon/helpers/sum.js

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+import { helper } from '@ember/component/helper';
+import { add } from './add';
+
+/**
+ * Alias for {{add}} helper
+ *
+ * ```hbs
+ * {{sum a b}}
+ * ```
+ *
+ * @param {number[]} numbers A list of numbers to sum
+ * @return {number} The sum of all the passed numbers
+ */
+export function sum(numbers) {
+  return add(numbers);
+}
+
+export default helper(sum);

app/helpers/sum.js

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+export { default, sum } from 'ember-math-helpers/helpers/sum';

tests/unit/helpers/sum-test.js

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+import { sum } from 'dummy/helpers/sum';
+import { add } from 'dummy/helpers/add';
+import { module, test } from 'qunit';
+
+module('Unit | Helper | sum', function () {
+  test('works as alias to add', function (assert) {
+    const input = [20, 10];
+    assert.strictEqual(sum(input), add(input));
+  });
+
+  test('works as alias to add for multiple arguments', function (assert) {
+    const input = [1, 2, 3, 4, 5];
+    assert.strictEqual(sum(input), add(input));
+  });
+});