Add is_json function

[RaeesFatima]

Validate if string is valid JSON.

Examples:
print(is_json('{"name": "Alice", "age": 25}')) -> True (object with string + number)
print(is_json("{name: 'Alice', age: 25}")) -> False (keys must be in double quotes, strings must use double quotes)

Instructions:

1. Create the function in validators.py
2. See and follow the used pattern
3. Write test cases.
4. Please do update the documentation too, github link: documentation link