input: Write a function to count the most frequent word in a text file.
  assert count_most_frequent_word('sample1.txt') == ('the', 5)
  assert count_most_frequent_word('sample2.txt') == ('hello', 3)
  assert count_most_frequent_word('sample3.txt') == ('world', 7)
output: def count_most_frequent_word(filename):
      from collections import Counter
      with open(filename, 'r') as file:
          words = file.read().lower().split()
      word_counts = Counter(words)
      most_common_word, count = word_counts.most_common(1)[0]
      return most_common_word, count