Add Dart solution for Valid Parentheses

[rrbharath]

Description

Adds a Dart solution for Valid Parentheses.

The approach uses a stack to track opening brackets. When a closing bracket is found, it checks whether the most recent opening bracket matches the expected pair.

Dependencies: None.

Time Complexity: O(n)
Space Complexity: O(n)
Difficulty: Easy
Tags: String, Stack

Put check marks:

Have you made changes in README file ?

• Added problem & solution under correct topic.
• Specified Space & Time complexity.
• Specified difficulty level, tag & Note(if any).

README was not changed in this branch.

How Has This Been Tested?

The solution was reviewed against the expected LeetCode Dart method signature and common sample cases.

• Verified valid examples such as "()" and "()[]{}"
• Verified invalid examples such as "(]"

Make sure all below guidelines are followed else PR will get Reject:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code so that it is easy to understand
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules