algorithm: first unique char in a string (#1103)

* feat: Added Algo first unique char in a string.

* Optimised algo to linear time complexity

* removed double quotes

* Review changes: if-else logic

String/test/FirstUniqueCharacter.test.js

@@ -0,0 +1,9 @@
+import { firstUniqChar } from '../FirstUniqueCharacter'
+
+describe('firstUniqChar', () => {
+  it('locates the index of first unique character in the string', () => {
+    expect(firstUniqChar('javascript')).toEqual(0)
+    expect(firstUniqChar('sesquipedalian')).toEqual(3)
+    expect(firstUniqChar('aabb')).toEqual(-1)
+  })
+})