Add Dart solution for Sqrt(x)

[rrbharath]

Description

Adds a Dart solution for Sqrt(x).

The approach uses binary search to find the largest integer whose square is less than or equal to x.

Dependencies: None.

Time Complexity: O(log n)
Space Complexity: O(1)
Difficulty: Easy
Tags: Math, Binary Search

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 perfect square input such as x = 4
• Verified rounded-down result such as x = 8

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