Add Dart solution for Length of Last Word

[rrbharath]

Description

Adds a Dart solution for Length of Last Word.

The approach scans from the end of the string, skips trailing spaces, then counts characters until the previous space or beginning of the string is reached.

Dependencies: None.

Time Complexity: O(n)
Space Complexity: O(1)
Difficulty: Easy
Tags: String

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 logic with "Hello World"
• Verified handling of trailing spaces

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