input: Write a python function to add a new contact to a SQLite database.
  assert add_contact('Jane', 'Smith', 'janesmith@example.com', '67890') is None
  assert add_contact('Alice', 'Johnson', 'alicejohnson@example.com', '54321') is None
  assert add_contact('Bob', 'Brown', 'bobbrown@example.com', '98765') is None
output: import sqlite3
  
  def add_contact(first_name, last_name, email, phone_number):
      conn = sqlite3.connect('contacts.db')
      cursor = conn.cursor()
      
      cursor.execute('''
      CREATE TABLE IF NOT EXISTS contacts (
          id INTEGER PRIMARY KEY AUTOINCREMENT,
          first_name TEXT NOT NULL,
          last_name TEXT NOT NULL,
          email TEXT NOT NULL,
          phone_number TEXT NOT NULL
      )
      ''')
      
      cursor.execute('''
      INSERT INTO contacts (first_name, last_name, email, phone_number)
      VALUES (?, ?, ?, ?)
      ''', (first_name, last_name, email, phone_number))
      
      conn.commit()
      conn.close()