Added valid_anagram.java

[Sahil-Madhyan]

• I have added a Java program to check if two strings are anagrams of each other. An anagram is a word that is formed by rearranging the letters of another word, such as “anagram” and “nagaram”.
• The program uses an array to store the frequency of each letter in the strings and compares them. It returns true if the strings are anagrams and false otherwise.
• The program also has some examples and constraints from LeetCode.
• Please review my code and let me know if there are any issues or suggestions. Thank you.