Add Binary Search Tree (BST) data structure implementation

- Complete BST implementation using R6 object-oriented programming
- Support all fundamental operations: insert, search, delete, traversals
- Include four traversal methods: inorder, preorder, postorder, level-order
- Advanced operations: height calculation, BST validation, kth smallest
- Tree visualization with ASCII art representation
- Comprehensive examples including student grade management system
- Create new Data Structures category for fundamental CS data structures

Author: piyushkumar0707

DIRECTORY.md

@@ -33,6 +33,9 @@
   * [K Folds](https://github.com/TheAlgorithms/R/blob/HEAD/data_preprocessing/k_folds.r)
   * [Lasso](https://github.com/TheAlgorithms/R/blob/HEAD/data_preprocessing/lasso.r)
 
+## Data Structures
+  * [Binary Search Tree](https://github.com/TheAlgorithms/R/blob/HEAD/data_structures/binary_search_tree.r)
+
 ## Mathematics
   * [Amicable Numbers](https://github.com/TheAlgorithms/R/blob/HEAD/mathematics/amicable_numbers.r)
   * [Armstrong Number](https://github.com/TheAlgorithms/R/blob/HEAD/mathematics/armstrong_number.r)