feat: Add Question 479 - Purpose of the .at() method

[Gunashekar31742]

I have added a new question (No. 479) regarding the .at() method (introduced in ES2022).

Changes Made

• Updated the Table of Contents to include entry 479.
• Added the question and answer section at the end of the README.
• Explained how .at() simplifies accessing elements from the end of an array or string using negative integers (replacing the old arr[arr.length - 1] syntax).
• Included code examples comparing the old vs. new syntax.

This adds value by covering modern JavaScript features often asked about in interviews.

[Gunashekar31742]

@sudheerj sir, i have added 479 please check it once

[sudheerj]

@Gunashekar31742 Thanks for PR. The PR included with some other question. Also, the answer of .at() method should be coherent with connection from one sentence to the other. Please improve it.