input: Write a python function to read a text file and return a list of lines in lowercase.
  assert read_file_to_lowercase('test_file.txt') == ['this is a test file.', 'it contains multiple lines.', 'each line will be converted to lowercase.']
  assert read_file_to_lowercase('empty_file.txt') == []
  assert read_file_to_lowercase('mixed_case_file.txt') == ['some lines are in uppercase.', 'some lines are in lowercase.', 'some lines are mixed.']
output: def read_file_to_lowercase(file_path):
      with open(file_path, 'r') as file:
          return [line.strip().lower() for line in file]