Add p0014: Longest Common Prefix

[MNnazrul]

Problem

• Number: 14
• Name: Longest Common Prefix
• Difficulty: Easy
• LeetCode Link: https://leetcode.com/problems/longest-common-prefix/

Changes

• Solution file added in src/
• Module registered in src/lib.rs
• Tests included with LeetCode examples
• Tutorial added in book/src/problems/
• Entry added to book/src/SUMMARY.md
• Row added to problems table in README.md

CI Checks

• cargo fmt --check
• cargo clippy
• cargo test
• mdbook build