feat: add comprehensive Data Structures concept page with tests

- Cover built-in structures: Array, Object, Map, Set, WeakMap, WeakSet
- Implement Stack, Queue, Linked List, BST, and Graph with BFS/DFS
- Include ASCII diagrams for visual explanations
- Add time complexity tables and trade-off comparisons
- Cover common interview questions (Two Sum, Valid Parentheses, etc.)
- Add 59 tests covering all data structures and edge cases
- Fix indexOf comment accuracy after unshift operation
- Clarify Object property order behavior in ES2015+

Author: leonardomso