merge: Add lower (#863)

String/test/Lower.test.js

@@ -0,0 +1,9 @@
+import { lower } from '../Lower'
+
+describe('Lower', () => {
+  it('return uppercase strings', () => {
+    expect(lower('hello')).toBe('hello')
+    expect(lower('WORLD')).toBe('world')
+    expect(lower('hello_WORLD')).toBe('hello_world')
+  })
+})